I think they pin everyone's application to a wall and then throw darts to decide whose they should look at next. They're all over the place - people without "wow" stats have gotten interviews a day after submitting the secondary while the wowsers are still waiting. Confusing. Note: I am FAR from a wowser.
EDIT: I didn't think I had to clarify, but apparently I do: I don't really think this is how their application review process works. I know that they have some rhyme and reason to how they do things, it's just some crazy algorithm we'll never understand. Nothing wrong with that. It keeps us guessing, not unlike an M Night Shyamalan film back when he made decent movies.