She just cant blantely forget everything about her first five years of her life. Thats hardly impossible unless amnesia occured or someone mindwiped her.
Actually, that's not hard at all to believe. Scientists today have figured that people cant remember much about their life from before age 5 or so. If you can, then it is mostly some big event such as a birthday or family member dieing. If you remember more, then you have an amazing memory.
Back on topic, I'd be extremely pissed if all of this turned out to be some dream sequence that goes back to the creation of the universe lol. Right now, that's the kind of vibe i'm getting from BBS. What strikes me as odd is that if BBS is 10 years before KH1, why there weren't any hints or strange occurances relating to it. This also makes me wonder if something cataclysmic happened to everyones memory or somehow everyone forgot about said events.