Presuming some superior ability by Sequoia to choose wisely. If in this particular case Sequoia is already "ambulance-chasing", hanging around YC's alley door hoping to pick up strays, I presume they are relying on YC's expertise, and simply expanding the opportunity to invest in worthy startups YC could not affort to fund. Otherwise why aren't they running their own show? Not to criticise Sequoia; I've been in 2 startups they funded. But VCs are notorious for herd mentality.