Since Dean Nicholas sent a bunch of personalized letters to interviewed applicants already this year, he probably didn't personalize all the waitlist letters. Maybe he personalized the waitlist letters of those who didn't get a "thinking of you" letter because they didn't send a thank you letter or update letter through the application portal. That would be my theory- everybody gets only one personalized note during the cycle.
I have a friend who got a personalized waitlist letter and he had like a 40T & high GPA, but I don't remember him mentioning getting the "thinking of you" letter. I'll check with him and see if he did. I'm sure he thanked his interviewer but maybe he didn't upload it on the portal like they told us to do. Hmm..just a theory, anyways.
Edit: He did receive the "thinking of you" letter and it asked him to consider sending an update (that variant of the letter, deliberated a few months ago). Well, there goes my theory. New theory- the waitlist is
secretly tiered and personalized waitlist letters are more promising but not the be-all-end-all, as mmmcdowe eluded to. Maybe only a subset get it personalized, kind of like those invited to Penn Preview on the waitlist (top 50 out of 500 on Penn's waitlist, or something like that)
We could go on and on about theories, but we'll never know unless somebody calls and asks.