Could it be that 4chan tracks its users, builds profiles on them, and serves them personalized content? For instance, that certain posts are displayed to a specific user only after a significant delay, or that the thread order isn't actually based on the last bump, and so on?
Probably not. The 4chan source code has already been leaked, and I haven't come across any mention of a feature like that. But I suppose it would not be too difficult to implement, there are already some imageboard scripts that let threads be tagged, so organizing them according to interest and user satisfaction feedback should not be a big challenge.