Waitlists are the schools way of fine-tuning their incoming class. Their enrollment rates are reasonably predictable, especially for medical school. My guess is that they straight-up accept enough people that, after the enrollment % is applied, would theoretically fill 80-90% of the class. That way, if more than the usual enrollment rate enroll, they have a 10-20% buffer. If fewer than the usual rate enroll, then no biggie. They then can pull as many people off of the waitlist as they need to get exactly the number in the class that they want.
A few years ago several undergrad institutions had far too many acceptees enroll than they expected. It put a huge financial strain on the institutions (Dartmouth was one) and there wasn't physically enough room to board or teach the incoming class. Dartmouth, I remember, had to put people up in hotels and teach classes in unusual places. I think adcoms all over academia responded by making that buffer even bigger, which translates into fewer straight acceptess and higher waitlist turnover.
-dope-
