"Uh...no. That isn't wha' happened. I got trapped here after we opened up the void to trap the Cybermen and the Daleks. But we did come here once by accident. And we took on the Cybermen the first time. Yeah I'm human." Rose answered and looked at him curiously. He hadn't said that before.

 In fact he drove in the fact that he was all alone by the end of their first trip. Did his regeneration do something worse than just forgetting her? Or was it best that he didn't realize that he was the last? Not knowing enough about his past but knew not to mess with time or risk Reapers coming was pretty sure to keep her mouth shut.