Unranked waitlist either means one of two things. The first meaning is that they are bull****ing you and they really do have some kind of ranking scheme they aren't telling you about just because they don't want people asking. The second meaning I've heard (the only one that makes sense) is that they have a certain class diversity they want to maintain, and when someone drops they go back to the waitlist pool to find someone like the last person. I can't imagine they consider EVERYONE again for each spot.