From my understanding, a lot of schools use “institutional needs” as opposed to a straight up numbered list. I think of it like this:
Everyone on each tier is basically equally qualified. Any numbering would be pretty arbitrary. So instead, in order to ensure a diverse class and a good mix of perspectives, they try to match the person who withdrew with a pretty similar candidate. For example, an OOS rural health focused candidate withdraws, they look for the closest match to that person on the tier to replace them. It lets them keep an OOS rural health focused candidate perspective within the incoming class which it’s important for ensuring diversity.
I could be 100% incorrect. This is just a p common explanation for “unranked” waitlists I’ve seen and it makes sense to me as opposed to splitting hairs trying to rank basically equally qualified applicants.