I also interviewed at South U and while I was there I spoke to some of the current students about being accepted, waitlisted, denied etc. They told me that b/c South does rolling admissions and it is still early in the application cycle they often waitlist people to wait and see how strong some of the later applicants are going to be. The girl I was speaking to said that she was admitted this way and she says many people are.
It is my impression that everyone who is good enough to be acccepted is interviewed and then after the interview the top applicants are admitted immediately, the ones that arent good enough are denied and the ones that probably are good enough to be accepted are deferred or waitlisted.
I would not worry because they did call you for an interview so clearly you were good enough to make it that far. If you have any questions I'd be happy to answer them because I also interviewed there.
Best of Luck!