Part of the fund is held for doubling-down in subsequent rounds of seed/series-a companies that are doing well.
And this is another brilliant part of Dave's model. He can rapidly apply lessons from the last 50-100 startups he funded to the next 50. He can rapidly collect metrics that will allow him to determine which companies are worth following on with.
His "sample size" is much bigger and therefore allows him to mine out statistically significant data about where to follow on.