WL can have many reasons. The best is they really want the kid but something like geo diversity or getting balance in majors affected a last minute decision. They want him/her enough to keep them on deck, to see who doesn’t matriculate.
Can’t assume there’s something lesser about WL kids.
Of course, there are kids they have no intention of ever taking from the WL. (A different topic.) That doesn’t speak to the kids they do want lined up. They cannot take every great kid first pass.
There are few absolute facts. It’s not like they have one set of expectations for ED/RD and are willing to take the dregs in Spring.