<p>Schools know that some of the people whom they accept will choose to go elsewhere - practically everyone applies to more than one school, and most students are probably accepted at more than one. If a school wants to get a class of about 1000, they might normally have to accept 3000 or so, depending on past experience to figure out how many are likely to actually enroll. If they guess wrong, they may have more than expected. If they underguess, they might be able to make it up by having a waitlist.</p>