In some cases it is due to simplification of complex issues (the American Revolution was about taxation without representation), in some it's moralistic fairy tales (George Washington and the cherry tree), in many it's dues to outdated texts and elementary teachers ignorant of history. In the case of a teacher who said Columbus landed at Plymouth Rock, I'd say it was because they were an idiot. BTW, Columbus never visited the future U.S.

Is there any chance that you've combined things in your head and never questioned them or checked your understanding of things?

Because it's easier to teach what has been taught for decades than to start teaching something different. I don't know what history teachers learned in college 50 years ago, or even 10 years ago, but now they're learning (at least where I went to college) what really happened (or at least what we think really happened, I guess it could change). University level history courses can be a rehashing of what you learned in high school, at least the general education ones that everyone takes, but if you start taking upper level history courses then you're going to learn a lot of stuff that you probably didn't learn in high school.

When I went through High School (2005-2009) I was not taught that Columbus discovered America, but that is what I was told when I was in elementary school. Also, the way that the Revolutionary War is taught, even up to High School, still would have you believe that the Colonists did it almost entirely by themselves, yeah you're probably going to learn that the French helped, but you won't hear anything about the Spanish or the Dutch. There's a lot wrong with the education system today, especially when it comes to teaching history.

There are a lot of books on the subject. One I would definitely recommend is "Teaching What Really Happened" by James Loewen. It was required reading when I did my HIS 300, which is where we learned how to properly cite and do academic research. So unless you were a history major, you probably weren't going to take it.

So, I'll be partially diverging from the US side by using some of my very little (just starting) experience teaching in Canada but a lot of it will be relevant.

1. It is incredibly hard to break away from what has been taught for years. An example here is how reservations are often looked at as being a case where the federal government is giving them money and such and it should be taken away because they are living on it as welfare. What these people do not realize is that we are living on their land and gained it by treaties which we honour in the form of those payments. Most do not understand the term "we are all treaty people" (basically Aboriginals get benefits for trading us their land and we benefit from having traded for it) because it has never been explained to them by their parents. What the teachers are left with is two options. They can spend a lot of time and effort fighting against these prejudices (and trust me, this would be a multi-week project for most misunderstandings) or they can go along with it and cover as much material as possible. Where I am located the former is becoming far more common as a result of a major push towards Aboriginal content and alternative ways of knowing.

2. Standardized testing: These tests are often created by people with little to no understanding of how things work in a classroom and instead make a test of 60 multiple choice questions. These tests leave no need for a 'deeper' understanding of the material which comes from going against the things you have mentioned by promoting a full education on a topic. instead they push for a 'shallow' understanding of the topics, an example I can find is from some Californian Standardized tests. in a single test you have questions involving the Japanese constitution, religious beliefs, world philosophies, Napoleon, the Industrial revolution, and so on (link below). There is simply no way to cover all this material without teaching the students barely anything about a topic before moving on and the easiest way to do that is to overly simplify it.

http://www.cde.ca.gov/ta/tg/sr/documents/cstrtqhssworld.pdf

3. Over simplification: This is a huge issue with history which I have already seen on multiple levels. I've listened to students talk about the Seven Years' War (French and Indian War for Americans) and while I could have systematically taken apart what they were saying with "while that's true it's also false because...." but it would have taken me hours to cover. Instead what happens is that we have to cover all of Canadian history (from pre-colonial to today) in a single semester and the only way to do this is to massively over simplify it. I am not joking when I say that this video was the entirety of the coverage of the War of 1812. The book "Readicide" by Kelly Gallagher stated at one point that to cover all the points many teachers are expected to school would need to be K-22. So to cut out those extra 10 years teachers are forced to cut a lot of information and teach a lot of things they know to be wrong but have to say because it's simply the only way to cover it in the time they have.

4. Bad Teacher Knowledge: This is a far smaller part and I want to be clear, I am not saying they are a bad teacher or anything like that, I am only saying that they may have been taught one way which has been proven incorrect. As a result they simply do not know any better and as a result are teaching the students incorrect information without knowing it. While there may be some that are intentionally bad I will not touch on that as they are an extremely tiny minority and I do not want to give the impression that all teachers are like them.

Oversimplification, plus a bias toward localism to connect the history to something more tangible and relatable. Check out the social science curriculums from the Mansfield, CT public schools.

• In kindergarten, it's community building with holidays and generic American heritage. So your exposure to colonial history begins with a solid dose of thanksgiving, pilgrims and construction paper handprint turkeys.

• First grade is an introduction to continents, and the stories of how the pilgrims and Columbus had to travel across a whole ocean. Maybe some of the kids get the two stories a little confused, but they probably at least come away understanding Europe is far away.

• Second grade is all famous people and how awesome it is to be a citizen.

• third grade is straight local history with field trips, reinforcing that New England and its history is what is most important.

• Fourth grade is native Americans, which begins to introduce some national-level material, but I'm going to guess they still focus mostly on local tribes. The Pequot Indian museum is just too awesome to skip.

• Fifth grade has "the northeast," which finally looks at English colonialism more broadly. I'm guessing this where they final mention that Jamestown was founded well before the Pilgrims landed. There are also sections on the West and Mexico, one of which presumably mentions that the Spaniards were in California even before Jamestown, but it's probably not a focus.

• Sixth and Seventh grades are world and ancient history. Eighth grade is the American Revolution on.

So now you're 12 or 13, and your exposure to European colonialism in North America comes from some community-building myths at age 5, some cool field trips at age 8, and a couple homework assignments at age 10 that you almost certainly rushed to finish.

Under those circumstances, it seems reasonable people later learn they didn't understand something correctly, even without adding to the equation teachers who may not be historically minded themselves and who are worried mostly about your reading and math scores.

I actually give the curriculums credit for trying to frame the issues in terms of skills and critical thinking, like using sources, etc. However, most people think of history as facts to be learned, and facts aren't something that build on each other or that you can improve with practice, like reading or math. So you hit high school or college and all you see is that you learned the facts wrong, not that you picked up skills to think about historical events critically (assuming you did, of course.)