My mum tried pointing out that she wasn't a stereotypical female as well, but I didn't push it too much and try to explain because I'm too young to medically transition in the UK and I've got too much to lose by trying to just socially transition now. I think that if I try to explain properly when I'm older it will be possible to explain to them, so my situation's a little different to yours. If my dysphoria got a lot worse, and I couldn't cope even with my girlfriend around (who somehow inexplicably makes me feel alright even though I'm not out to her) then I would have to make a start on things and explain it to my mother, probably by explaining how it's not just that I don't fit in with females, but that I don't fit in with my physical body, and that I'll just feel a whole lot happier as a male, as me.
Edited for personal info.