<p>when you reject a college?
First of all, colleges over-admit, because they know they won't have a perfect yield. It would be a disaster if everyone admitted went. Even if someone declines an offer of admission, that doesn't mean that someone from the waitlist gets in. Some colleges never go to their waitlist.</p>
<p>It is a bit silly. You don’t ACTUALLY clear up a spot on the waitlist. It does make some sense, though, because the more people that decline a school, the more likely the school is to go to its waitlist. So each person who declines an offer of admission brings the school a bit closer to opening a spot on the waitlist.</p>
<p>The sum of declines form an aggregate which allows colleges to dip into the WL. Sometimes this number is big enough, sometimes not. That simple.</p>