Perhaps, CMU is an anomoly. CMU admits students to specific colleges, each of which has a very specific enrollment number. CMU also waitlists students to specific colleges.
Well prior to May 1, CMU already knew which waitlisted students for a specific college indicated that he/she wanted to be placed on the priority waitlist for that college. If a student opted to be on the priority waitlist, that student committed to deposit $800 within 72 hours of admission.
They also know which waitlisted students are not seeking substantial financial aid, such as merit scholarships (because waitlist admission at CMU is not need blind and there is only minimal financial aid available for waitlisted students).
In all likelihood, CMU’s admission staff is divided into small teams, on a college by college basis to fill the open spots for each college. From there, these teams read the essays of top tiered waitlisted students, for each college. These essays will help separate and differentiate individual students that truly want to attend CMU. And, by the end of the day, CMU will be able to fill the open spots.