OK, here's my guess as to how they're making decisions at Cornell. I think they have a big pool of applicants they've interviewed and are considering, and a target number of acceptances to issue in order to get the class size they want. Right now they're in the process of whittling down the pool to the target number, and as they make each decision to remove an applicant from the pool, they are sending that person a waitlist letter. So their class is ALWAYS tentatively full, because they have more than their target number in the pool of potential acceptances, even if they haven't issued a single actual acceptance yet.
I've sat on an admissions committee before, and this is one of many ways to handle the process. It's also the only one I can think of that fits all the info you all have reported here. It works well for schools in the top echelon, although usually when it's done, all the letters are sent at once.
