Well, that is because you are missing a huge part of the Maya calendrical system. What you are describing is just one element of the Maya Long Count, one of three calendars used by the Maya people. The other two are the Haab', 365 day solar calendar, and the Tzolk'in, the 260 day ritual calendar.
The Maya Long Count is a little different than the solar or ritual calendar. Rather than keeping track of a cyclical period of time, the Maya Long Count is a running total of the amount of days since creation. The best way to describe it is the temporal equivalent of a car's odometer. It's just always counting up since when it started. This running total is typically kept track using five numbers. We write these numbers out as #.#.#.#.#, but for the Maya they would have written them out in glyphic form.
The first number is the k'in and it occupies the position farthest to the right denoted by a 0 (#.#.#.#.0). The k'in essentially keeps track of the number of days in a month. After each passing day another number is added to the k'in position (#.#.#.#.1, #.#.#.#.2, #.#.#.#.3, ...#.#.#.#.19) until the k'in reaches 20. When the k'in reaches 20 it resets back to 0 and the next position is increased by 1 (#.#.#.1.0).
This next position is called the winal and is second from the right (#.#.#.0.#). Like the k'in, the winal increases, as well. But it only increases by 1 every time the k'in reaches 20. This conforms with the Maya solar calendar of 18 months consisting of 20 days (plus 5 extra). And instead of resetting to 0 at 20, the winal resets to 0 at 18 in order to reach an approximate solar year (#.#.0.17.# --> #.#.1.0.#). However, the extra 5 days in the solar calendar to make a full year are not recorded in the Maya Long Count. This causes things to drift over time.
When the winal reaches 18 the next position increases by 1. The middle position is called the tun (#.#.0.#.#). Like the other positions, the tun increases by 1 and resets at 20, but it requires the winal to reach 18 before it increases by 1. That means, almost 1 year needs to pass before the tun increases.
The next position, second from the left, is the k'atun (#.0.#.#.#) which you are asking about. The k'atun, like the others, increases by 1 every time the position under it reaches 20. Another way to think of it is that the k'atun consists of 20 tuns or 7,200 k'ins (days). You could think of the k'atun kind of like a decade in our calendar in that it is a period of time greater than a year. It allows one to easily talk about time.
The reason you see this variety of k'atun numbers associated with different years in our calendar system is that you are not seeing the last position, the b'ak'tun (0.#.#.#.#). The b'ak'tun provides context for the k'atun. Think of it like this, someone could be talking about something that occured in the the 40s. But, is it the 1940s? The 1840s? The 1740s? The 640s? You don't know because there's not enough context. The b'ak'tun provides that. And like the other positions, the b'ak'tun increases by 1 when the position below it, in this case the k'atun, reaches 20. Each b'ak'tun consists of 144,000 days or approximately 394 solar years. You can kind of think of the b'ak'tun as the equivalent of a century in how you talk about time.
Of course, there are more positions in the Maya Long Count, but they are rarely used. That is because currently we are in the middle of the 13th b'ak'tun. In fact, today's date is 13.0.5.5.1. That means only 1,873,901 days have passed since the start of creation (August 11, 3114 BC). When the Maya do talk about these other positions, they are typically in the context of future anniversary dates of an event like someone ascending the throne. You could think of it like the U.S. celebrating their independence day on July 4th in the year 3814 (if the U.S. still exists at that time).
Finally, the Ahau part is telling you which day it is in the Tzol'kin. Each day in a month has a separate name and each month has its own name. The 1st of the month is Imix', the 2nd is Ik', the 3rd Ak'b'al, etc. Ahau is the 20th day name in the Maya month. And because of the drift within the Maya Long Count, the day number of Ahua will not sync up the same every year within the Maya Long Count. For example, December 8th, 1263 is 11.2.0.1.0 9 Ahau. But, December 8th, 1264, just one year later, is 11.2.1.1.6 11 Kimi. And Kimi is typically the 6th day of the solar month. You can see there is a drift of 5 days because the tun only consists of 360 days and not 365 days.
I hope this clears up your confusion and if you have any other questions, please let me know.