Right on with your guess :) And I agree with "it guarantees good content" part. However, one point we didn't mention is the staleness of the global bucket. For a generic search "funny" on Youtube, most results are 4-5 years old.

We want to focus on upcoming talents, which means we want to bring in new content to the mix, to some people, "fresh + good" is better than "best", and I believe that's why StumbleUpon model is successful.