JP is correct. Schools -- med schools, colleges, law schools, anything -- know that not all accepted applicants will eventually enroll. The number of acceptances that translate into a certain number of matriculants is known as a "yield." While the yield will vary from year to year, the change is slight and usually inconsequential. Airlines sorta overbook on a regular basis too, I believe, for this very reason.
Anyway there have been instances in the past where a school projected its yield to be lower than what was seen at the end of the application cycle. This happened to Cornell, which accepted way too many kids. They offered those students a free year of med school if they just waited for the next class. Surprisingly, NOT EVERYONE took it.
Weird.
W.