I'm also on the waitlist and can only speculate.
I think the first round of waitlist offers is mostly given to highly ranked waitlisters (usually OOPs given their pre-interview cutoffs) and also internal movement (i.e. people who got initial offers to their 2nd/3rd site, typically IMP or SMP, getting offers to their first choice, typically VFMP). During the second round of waitlist offers (after most of the internal movement), I anticipate a significant number of spots will open up for the satellite sites, where "lower" ranked waitlisters (hopefully us IPs) will be offered these spots. An