@CarnegieDad: Your question is answered at length in this thread, and you don’t have to go back too far. Search the thread for the word “rank” or you may want to start with this post and this one which will tell you pretty much everything you need to know about how BS waitlists work.
The short answer is that WLs are there for the benefit of the schools, can be quite deep, are not used until a school finds itself underenrolled and, for the “top” schools, underenrollment rarely happens. When a school needs to go its WL, it will be to select an applicant who best matches the profile of a student who chose to matriculate elsewhere. For example, the open slot might be for a full-pay female day student who is an impact volleyballer. Basically, each BS crafts a class and uses the WL to fill very specific holes that occur between A10 and the start of the new school year. The selection is not based on ranking, but on best-matching.