So what I've noticed is that applicants who applied in mid-Jan through March had a small amount of As. No reported As from the bunch of decisions on 4/30 (but I'm sure we're just looking at a small sample size) According to their accepted students Facebook page, they only have 164 members (obviously not everyone joins so I'm sure this number is higher as well) If I had to guess, I bet Penn just wanted everyone to have a decision by that 4/30 date so they waitlisted the majority to take stock of their class size. I know some other schools follow this pattern as well: interview until March, don't accept anyone from Feb-March so everyone has been interviewed before taking anyone off the waitlist.
Long-winded way to say I don't know anything lol