More importantly, it depends on how early you interview. If you interview late in the year and only a few seats remain, your interview had better be pretty good as your chances are far slimmer simply because adcoms become relatively more picky.
Analogy to consider:
Imagine you had $1,000 in one dollar bills and your friends started asking you for money one dollar at a time. The first few times you'd be like, yeah ok here you go I have 1,000 dollars, it's no big. However, when you are down to your last two or three dollars you're going to be pretty particular about who you give those dollars to and for what they are to be used for.
Also, just because the school interviews 400 and takes 100 doesn't mean you have a 1/4 chance and that 1/4 of the people there on your interview day will get accepted. One interview day they could take zero and then another day could take 3/4 of the people. Taking 100 out of 400 simply means that after all of the interview days, 100 people are accepted--it says nothing about your odds on your particular interview day. Just focus on kicking ass rather than odds.