Sorry for being not clear, let me try again:
I assume YC is presented with a glut of choices, there would probably be a triage stage that runs something like this:
1) nonsense (say 60%)
2) maybes (say 20%)
3) positives (say 20%)
Of the positves YC can due to resource constraints only accept a certain number, say for the sake of the argument that YCs resource constraints allow it to accept half of them, or 10% of the original group.
If after that you follow the 10% that got accepted with the 10% that did not as a control group then I expect both groups to do equally well in terms of success, in other words the 'hit rate' should workout roughly the same in both groups.
YC will find that it has invested in some duds, most of them will be so-so and there will be a very limited amount of take-offs, in the other group the ratios will be very similar.
I hope that makes it more clear...