"At Y Combinator, our goal is to get you through the first phase. This usually means: get you to the point where you’ve built something impressive enough to raise money on a larger scale. Then we can introduce you to later stage investors—or occasionally even acquirers."
What I was trying to get at above was that if "% of companies funded" post demo-day is a metric YC wants to maximise, then it would be trivial for them to get to near 100% by only selecting the obviously investable propositions 3 months earlier. If they did this, they'd simply be following whatever fad that VCs happen to be chasing and might miss outliers (even PG tried to convince the Airbnb founders to try something else).
"At Y Combinator, our goal is to get you through the first phase. This usually means: get you to the point where you’ve built something impressive enough to raise money on a larger scale. Then we can introduce you to later stage investors—or occasionally even acquirers."