Some extreme math to make a point about admit rates....

Two Colleges, Apple and Orange each have 10,000 applications for 2,000 freshman seats, and they both have a theoretical yield in regular of 25%.

Apple University doesn't think Early Decision is the right way to go, and so their 10,000 applications are all RD apps. They need to accept 8,000 students to get 2,000 to show up, so Apple's admit rate is 80%.

Orange College is really focused on admit rate-their President puts pressure on the Dean of Admissions to get the admit rate as low as possible.   They aggressively market ED1 and ED2, and give the benefit of the doubt to all ED applicants on the fence.   Through both ED rounds they get 2,000 applicants, and accept 1,500 of them.  (75% ED admit rate)   They now have 8,000 RD applicants, and 500 spots to fill.   Remembering this is an extreme example, they accept 400 students in RD, yielding them another 100 students.  (5% RD admit rate, before WL admits)  Now, they know that if they call their waitlisted students and make sure they're going to come if they admit them, they can roughly get 100% yield from their WL.   So they admit 400 more off the WL, getting them to their 2,000 seats, and pushing the overall RD admit rate up to 10%.   At the end of the day, they admitted 2,300 students, and Orange's admit rate is 23%.

Again- these aren't real examples, and I simplified some of the many moving parts of enrollment management.   But even in more real life examples, ED and Waitlist strategy can wildly influence admit rate.  It's just as much a picture of the Dean's enrollment philosophy as it is school quality.   We all know about the rise in ED, but I feel like I've noticed the Waitlist (or ED3) trend more in the past few years, and it gets less discussion.   Instead of colleges using the WL in case they don't hit their number, they are specifically planning to use the WL to hit their number.   There can be more reasons behind this than just yield, but as more colleges pull this lever purposely, the April/May trickle down impacts of massive WL activity will be significant.