Here's some extra tidbits of info, it could be right or wrong, I'm stating all hearsay.
USC interviews a lot of people because they put more emphysis on the interview rather than the application stats.
The early round of interviews are usually the cream of the crop in terms of GPA. Meaning the majority of people invited have high GPA's and something extra.
Later interviews are for people with lesser GPA's but your app looks good in some places, so you might get an interview in Jan/Feb/etc. OR you have a high GPA but you turned in your app late, closer to the deadline.
Someone posted that they got an interview even though their app submission date was 9/27 or so, given that, if you got a email saying that your app is complete, your app was considered for an interview for Nov but didn't make the cut this time.
Things to be happy or sad about:
If you turned in your app early, in August and you have a high GPA and you didn't get an invite for the Nov interviews. You should be slightly worried. You got skipped over this round, not the end but they still skipped you for whatever reason.
If you got an invite be happy but remember they are inviting 599 other people for a 100 slots or so, so getting ready to smooze and make an impression.
At least for USC, they give you a chance to make an impression at the interviews rather than turn you away without a fighting chance.