If you look at the chairs it's the same chair. What if Jenny crash landed on the Dalek asylum and the fall caused her to regenerate? Also, sense she is a Time Lord, what if she somehow figured out how to time travel? It would explain why she is with him at different times. And as for the forgetting....... Maybe she has a forget button in her ship?<--- I like this theory
Marvel Comics Writers Explain Why They're Making Thor a Woman || Basically, Thor for some reason can't be Thor (but will still be a character) and a woman takes up the mantle (uh hammer) of being the next Thor.