Some schools just don't bother ranking their your position on the waitlist (hence the term "unranked waitlist.") In this case, it's somewhat random who gets taken off the waitlist first.
I heard a theory where if an accepted student withdraws, they take someone from the waitlist whose profile somewhat matches the original student's. For example, if an accepted student who happend to be a URM with lots of research happened to withdraw, they might try to find another URM with lots of research from the waitlist. Of course, this is just a theory and there's no way to know for sure it really works this way at any school.
Also, even if a school has a rank, they might be unwilling to tell you exactly what number you are so they can be somewhat flexible on their side. If they tell you flat out you're #15 on the waitlist and somehow you happened to have a friend who was #17 who got off the waitlist before you, you could cause some problems.
You also have to consider the rank can change as more people are added to the waitlist both in front and behind you.