They invest relatively little compared to the successes, to the point that they're even using SAFEs to double down. The thing is YC can't invest more; too much money is bad too for early stage. So it makes sense to invest into company with lower odds. They've also said that some of their best outcomes were just slightly above the bar, so there's probably a lot of potential unicorns slightly below the bar as well.
https://www.axios.com/2024/04/06/ycombinator-funds-alumi-inv...