The USMLE is a comparative exam. It's not about how many questions you get wrong, but about how you compare to others who are presented with the same set of questions. If you correctly answer many questions that most people get wrong, your score rises considerably. If you mess up in questions that most people get right, the opposite occurs. So to sum it up, in order to score well, 99 that is, you have to:
1. Get difficult questions right.
2. Not get easy questions wrong.