I was luckily enough to interview at a program where the PD advocated transparency. Therefore at the end of the interview day, he told us exactly how we are ranked. Here's the overall idea of what he told us:
1. All interviewees are given a numeric score from 1-100.
2. a Max of 30 points is given to your interview. If your interview was average, you get 18 points. for exceptional interview answers, you get more points, but to a max of 30.
3. a Max of 30 points is given to your transcript. If you were average student with Pass and HP grades, you get 18 points. For every Honor in THIRD year only, you get extra 2 points.
4. The rest of the points are given for letter of Rec, (exceptional letter would get 2-3 points, average letter gets 1-2 points, etc.), research, CV/community experiences, and others that I can't remember.
Then he told us if you have a score of XX, you'll most likely match there. If you have score of YY, you'll not match there. the scores in between XX-YY is the gray area that varies each year.
So in summary, it seems that your interview only matters for a small percentage. Even if you dazzled the PD or Chairman, the likelihood of a match is still mostly determined by your paper application. Of course this is just one institution for one specialty, so take it with a grain of salt.