That is a great question. I didn't find any theories. But, what I think is that, the  Mayans just had a better grasp on astronomy than the Aztecs did. Since the Aztecs calendar derived from the Mayans calendar, maybe the Aztecs misread it? They could have changed things here and there so they could better understand it. It is not known how the Aztecs discovered the Mayan calendar, it could have been word of mouth which could get misconstrued. Honestly, I have no idea, but, that is something I definitely want to look more into.