When I called admissions today,
I specifically asked if the letters can be acceptances/waitlists/rejections, and the woman I spoke to said yes. Although the admissions people have been wrong before, I think this is consistent with my theory that UICOM waited for seats to open up on May 2 before making final decisions on the remaining applicants.
What worries me is how many "extra" people they accepted in anticipation of people withdrawing seats. Who knows what the proportions of accept/waitlist/reject will be.
edit: I just checked my blood pressure and it's 130/74