That's... a good question. I think AX's memory was a huge mess. First of all, between Xehanort and Terra, there's two sets of memories and two different personas stuck in one guy. Then, you have that one time when Xehanort tried to get rid of the Terra persona left in there by poking it out with his Keyblade, which seemed to mostly seal both of them away.
I don't think that AX was truly faking it. That's not to say that bits and pieces didn't rub off on him, like the "superhuman talents" Ansem the Wise observed, a fascination with the hearts and the darkness, and (from what we've seen from Xemnas) a small recollection about Aqua and Ventus. Down-in-there impulses that couldn't be removed with a memory wipe.
Probably thanks to some time and Ansem's testing, more of those impulses began to resurface til AX was able to piece it together as a memory of sorts. I think some of his more dubious actions, such as the first scene of DDD, were results of that subconscious Xehanort persona getting the conscious self to act on instinct rather than calculated thought on his own part.