The way I see it, they have smaller (but frequent) company-branded bundles (eg, the FrozenByte bundle) and larger, more exciting numbered bundles (like this one - #4). The numbered ones always seemed to have more games, more hyped/well-known games and also sell a lot more.

Maybe I'm wrong, but that's how I view them. As long as the numbered bundles can continue to include some big well known games, them I think they can avoid "bundle fatigue". But we will see, I guess. Its still too early to know for sure how repeatable it is, though it looks positive so far - I mean, $900k in one day is pretty good!