I dont think rank is determined just by interview score, but I am also certain that you are not placed on a waitlist in the order in which you interviewed. In fact, people who get waitlisted earlier in the process when there are more seats available are probably weaker candidates than people who get waitlisted later when they're competing for fewer spots. For example, if you get waitlisted in December after a November interview, then you're probably not exactly what the school is looking for since its doubtful that their class is even half full, and yet you didnt get a spot. If you are waitlisted in April after a March interview, the school might just love you, but not have many spots left. Therefore, I would presume the April waitlistee is higher on the list than the December one.