milias
I wonder if the alphabetical order of your last name matters.
Since everyone has all there theories, This is my theory. The get all the applications and divide it among the 10 adcoms so each of them has say 100+ apps. The apps are random at this point and depends how deep you are in the pile. Talking with Booth before in the mock review session, they said it takes them about 30 minutes per app. You get your invite as soon as they get to yours and go interview or not.
I don't think there really is a sophisticate system where they alphabetize or break apart by major or XYZ. Visit a couple schools now, it just seems that most schools are pretty chaotic and just do everything randomly without crazy planning or conspiracies. :D
Also i don't believe Ross only send out once a week. I can look at last year to make sure, but I think it just flows through as soon as they say interview.