It depends on the schools you interview at. Some schools only invite you to their interviews if they're 99% sure they're going to accept you. So basically 99 out of the 100 candidates they invite get in after the interview. Other schools invite 200 students but reject (or waitlist) 100 of them. Often times your interview has very little effect on the final outcome (interview performance is very hard to grade). If they accepted to you, you probably looked very good on paper to begin with. If not, chances are you weren't exactly what they were looking for, and they wouldn't have invited you if it weren't for the fact that some candidates are going to turn down their offers and they need some back ups.
It could also be that you're completely bombing the interviews. Those are rare cases though. The interview process is not very difficult and most people do well enough.