Don't worry, that's completely normal. I would bet they don't go through them chronologically. In fact, it's just as possible they go through them alphabetically, numerically, or based on home state. There are a million reasons they could go through yours or mine before someone else's, and vice versa.
The more you try to figure out the process, the more you'll realize we have no idea how they work, and you'll realize it could just as easily be completely arbitrary--a lot of it has to be. You've already gotten two interview invites, one at a highly prestigious place--just relax and let the process play itself out. You seriously do yourself no good to be so neurotic, and it makes people who have received fewer interview invites than you--even zero invites--feel pretty bad when they feel like they have to worry this early on (when they really don't). You're a qualified applicant, you're going to get a lot of attention. Just be patient.
(To put things into perspective, I was complete here nearly two weeks before you--and I have no reason to be worried... if it were November or December, I might be concerned... In past cycles, most of my interviews came several months after I was complete)
EDIT: If you and I interview together or are accepted at the same school, I'm going to buy you a beer and make friendly jabs about how you were so worried...