I really don't think that they Mayan calender correctly predicts the end of the world for several possible reasons:
1. The Mayans basically locked a bunch of people in a room and told them to predict all events in the future. They might have just gotten fed up with their jobs and said that the world would end on X day, so that they could go back to their lives, hoping that day X would be too far into the future for anyone to remember that it was them who predicted it when the world presumably would not end.
2. The Spanish Conquistadors killed them before they got a chance to finish the calender.
3. They might have gone completely insane from being locked in the aforementioned room, and killed themselves before they could finish.
4. They might have gone completely insane from being locked in the aforementioned room, and were so delirious that they really thought the world was going to end.
5. Or maybe they were on drugs.
Either way you slice it, they probably weren't right.