The explanation that I've been given is that you're discussed after your interview and given a composite score. Then the people with composite scores above 90 (just an example, I don't actually know how this works) get accepted right away and the people with composite scores below (again I am throwing pretend numbers out here) 10 get rejected right away, and everyone in between gets put on 'hold'. So it really doesn't mean much. By April 30 you'll get another update that says accepted, rejected, or waitlist. And then May 1 the people on the wait list get their rank. Does that make sense?
Someone posted the excel spreadsheet of all the different years and how far along the waitlist they got above!
(open to any corrections from anyone who knows the process better than I do)