Yeah, seems to be completely random too. At least with this first batch, its random in order (people from September interviews and January/February getting their position letters) as well as decision (all three tiers and some acceptances as well).
So essentially if you got the “your high on our waitlist” additional letter you are getting straight up accepted (more likely) or top 1/3.
I’m going to say this to give people, whom are bleak on their position if already given it, some hope. I know a handful of people whom were accepted and have no intention on attending still holding positions, or have recently within the past few days given up those positions (at dates after this first batch was sent out). So hang in their and expect a decent amount of movement!