There is a lot of educated gambling going on. Many schools are competing for a small subset of the applicants. Some applicants are sitting there on May 15th with 4-10 offers of admission (and on the waitlist at other schools). So, on May 15 everyone must drop all but one offer (but may stay on an unlimited number of waitlists). A school may know that many of the fine applicants offered admission will choose to go elsewhere. Knowing that, they may make 200 offers of admission hoping to fill 100 seats. If on May 16, they have only 90 seats filled, the med school starts calling people who were placed on the waitlist. In the unlikely event that they have 102 bodies for 100 seats they'll make it work. In all likelihood, at least 2 will drop later when they get off of waitlists at other schools. The real problem arises when there are 20% more students than seats.... in one instance years ago, a school offered one year of free tuition to anyone who agreed to delay admission by one year. No dean wants to have to do that.