Just because the interview went superb alone doesn't seal the deal.
Many (if not most?) schools will consider your whole file at a committee meeting sometime after your interview.
There, they give you a score based on everything about you and your app - gpa,dat,personal,extras,hours, etc,...to compare with others that interviewed. So, the interview alone cannot determine whether you get in or not.
Chances are you probably did great at the interview, but most likely didn't score high enough at the committee meeting to get an outright acceptance versus those who did. Similarly, this is how post-interview rejections occur.
This is why, in most cases, an interview offer alone is nothing to be too excited about if your other stats are weak; you may not 'survive' the committee ranking arena to obtain an acceptance. However, if you are waitlisted, depending on how good your stats are you may have better odds as the cycle goes on.
