I suspect those numbers may be skewed by the same "we only hire the top 1%" phenomenon. There could very well be a recurring group of people with not so great ideas that repeatedly apply to incubators, and repeatedly get turned down, whether with the same idea or a new one.
I'd be interested to see the skew as well. Maybe a large part of those declined were unserious college students or similar.
Sure, at least half of the applications hit the trash bin. I recall a comment from pg at startup school in 2010 correctly, he said a surprising number of the applications to YC (I want to say 20%) were in ALL CAPS. That's an easy way to self-select one's way into the bit bucket. But even at half the size, there's still at least 1000 applications that are likely well-formed in one way or another. PG has said that the hard part is finding something exceptional in an application that would put it over the top.
I'd be interested to see the skew as well. Maybe a large part of those declined were unserious college students or similar.