I think the key is that you can get an interview invite at any point in the review process. If the first reader of your app loves you then he/she can invite you to interview before anyone else has read your app. In this case, after you complete your interview, it might take a LONG time to hear back because other adcom members still have to go through your application. So plenty of these candidates will end up being dinged down the line.
Or you could be invited to interview after your app was thoroughly read by a few adcom members. I think in this case, the adcom is already set on admitting you and is just waiting to see the outcome of the interview. If the interview outcome is positive, then you get admitted pretty quickly because there is nobody else left to review your application.
The people who hear within 2 weeks are probably the people in this later category.
(All speculation.)