She grew up with a time crack in her wall so she can remember. I'd also imagine being a time traveller would have something to do with it too.
- This is addressed in one of the Night and the Doctor mini-episodes (specifically the one entitled "Good Night"). Basically, it's a paradox and she'll have to learn to live with it, even though it disturbs her. In the mini-episode the Doctor gives her an object lesson in paradoxes by allowing her to encounter her younger self for a few moments (remembering their previous encounter happened in an alternate timeline so it doesn't count).