I think this is too narrow a range to determine where bugs are in the process.

Bugs in the application process can only really be rooted out by startups that apply and are successful.

Startups that apply and fail could point to a bug in a number of different areas - not just the application process.

If the purpose of YC is to make money and improve startups, than they are failing at both of those things if they pass on startups that eventually succeed.