Oral tradition is, by nature, fluid and often contradictory. In addition, supernatural beings are "untethered" to reality, so it is not possible to correct a story with scientific, empirical observation: if I say that a duck has four legs and roars like a lion, I will be quickly put in my place by someone who has actually seen a duck. On the other hand, if I am in a storytelling session in which I assert that a supernatural being of some sort takes on a certain appearance or originated a certain way, others in the room might say "well, I have heard it differently", but they will not likely say that I am wrong, because no one can be sure.

We can observe this is very real terms when pre-industrial rural Europeans would describe the origin of the fairies. Someone might recount how they were neutral angels who left heaven when Satan fought with God, but these angels were not bad enough for hell, and yet they were not allowed re-entry into heaven, so they were forced to walk the earth as "unaffiliated" supernatural beings. Another person, then, might tell the popular story about how one day God came to visit Adam and Eve. He arrived with little warning, so Eve was only able to wash some of her children and consequently hid the rest. God then said, "what you have hidden from me will remain hidden from you." And that is the origin of the supernatural beings, who are often called "the hidden folk." Then yet another person might assert that these entities were the souls of the ancient dead, who lived before Jesus and could not win Salvation. They lingered in the landscape, not as ghosts, but rather as entities in their own right.

That is how the folk dealt with this sort of thing, and a storytelling session might reveal any number of these explanations, even though they were contradictory.

Authors in the ancient world were drawn to the wonderful stories told in their oral tradition, and they were probably frustrated by these sorts of contradictions. If an author were a true believer, it would be easy to imagine this person attempting to find the "true story" behind the oral narratives. It was easy, then, to regularize the folklore, to write down the "definitive" versions of these stories. Of course, what one author might regard as the best story - the thing that was closest to the "truth" - might not be something that another author would settle on. The result is that primary sources from the ancient world frequently hint at the realm of possibilities, including the sorts of contradictions that are inherent in oral narrative, particularly when dealing with the supernatural.

It's also important to point out that modern books on "Greek Mythology" - particularly when they are popularized presentations - will strive to offer the same sort of definitive version. As a result, one's introduction to Greek (or any ancient) mythology tends to be by means of one of these simplistic "unified" mythologies. More sophisticated treatments of the subject will reveal a spectrum of primary sources that hint at the contradictions, which were likely far more complex and diverse in the actual oral narrative.