In my eyes, when I think of University prestige, I think of Universities that everybody wants to get into and dreams of attending. So a measure of prestige would be how picky the school can be because of the number of people who apply relative to the slots available. Another measure is how frequently would someone turn down an offer? In my eyes, the more prestigious an University, the less willing students are admitted would be willing to walk away for whatever reason. That is why I think this ratio when it is a large number indicates a lot of brand strength/prestige. Now of course there are ways folks can try to manipulate that, but the point still stands nonetheless.