Just a guess,
but some schools accept hypothetically 50-65% initially (leaning towards the upper % range ~65%) of their class pre-dec and waitlist or reject the rest of their pre-dec.’s / apps that they don’t want
obviously some students decline their seats, so this this brings their upper range % of students accepted down to say ~low 60%,
so, they continue to interview post-dec. and as the season goes on, they fill up seats/ppl give up seats/they reject students/etc. so numbers continue to fluctuate slightly, yet steadily increase as seats continue being filled.
By the almost mid-end of pre-dec., they look at the “haven’t interviewed list of potentials” for post-dec. invites. But come March. 1, when schools know which students hold multiple seats, they adjust accordingly/ look back at their waitlisted folk, review the post-dec interviewed ppl, etc.
After interview season is over (typically end of March, beginning April) they basically have 90-93% of their class filled and by end of Apr.-May it gets to 99-100%—possibly even 101% (basically the whole class is set with a few students like 1-3 more than the class size to account for last minute fall outs, etc). If that surplus ever dips lower than their said class size they pull students from their long-ass waitlist while continuing to reject ppl.
This is the typical routine of the average school. I believe some schools might even have 70-80% of their class (maybe even 85-90%) filled pre-dec. taking into account their class size, strength/popularity of their program (i.e. Columbia, Harvard, UPenn, UCLA, UCSF, etc.), sky-high tuition which deters multiple students, etc. and adjust accordingly for their school’s needs based on the infinite data from past records of their previous application processes to help them be extremely predictable, efficient, and accurate each cycle so as to prevent financial loss and wasted time.
But then again,
just my guess. 👍