I think it has something to do with BB's brain.
In chapter 223 where Luffy is competing with BB in the tavern in mocktown, Luffy says is there something wrong with your brain?
In chapter 225 There's another random person who comments on his brain.
I think BB's Darkness power allows him to extract DF's but he can only take a certain amount so he's picking the DF's he wants, such as WB's Quake fruit.
It's almost like he has multiple personalities and however many personalities he has is how many DF's he can have.
In Impel down when Luffy learns who BB is and BB asks something like, Oh, I didn't tell you my name back then. Did he just forget or was he under a different personalty and therefore didn't know if he told Luffy or not?
Just from re-reading all of BB's lines and watching his interactions with others, sometimes he seems completely insane, other times he seems childish and compitive in things that don't really matter, and other times he seems like an all knowing psychic.
I'll wait for Oda's explanation tho before making a final judgment about this. Be it personalities, his brain, his darkness fruit or just being a D or a mixture of all we can't really know for certain until then.