True, but many schools will call students on the waitlist and ask if they’d accept if offered, then offer only if the answer is affirmative. They get close to 100% yield on those admitted from the waitlist, which could affect their total admit rate. And a school that fills 50% of its class in the ED round with, say, a 95% yield, and another 10% of its class from the waitlist with a 100% yield, could quite dramatically reduce its overall admit rate through such shenanigans. If it was so inclined.