Think of it this way: schools do not order all applications in terms of desirability, and then go down the line sending offers until they reach a cutoff point. Instead, they put together a class, and they’re each trying to include certain elements that largely but not exactly overlap with other schools. They make their admission offers according to what they still need in the incoming class. They don’t want their individual applicants to be well rounded, they instead want the income class to be well rounded, with students who distinguish themselves each in a different way.
Maybe UVA or Princeton or wherever has a particular need for a certain unusual characteristic, to meet their goal of a well-rounded class. (I don’t know, a tall Jewish vegan goalie for the water polo team or something. Work with me here!) An application that has that unusual characteristic becomes the hot ticket for an admission offer. Maybe they already have more than they need of some other characteristic; an otherwise good application that relies on that other characteristic to distinguish itself may sit around gathering dust on the waitlist, even if their grades and test scores might seem “superior” to the first applicant.