I do this too. That's if I can manage any kind of coherent speech. Even if I have a chance to think about what I'm going to say it comes out an octave higher than I had rehearsed in my mirror.
I'm picturing the opposite of that video of the bloke dressed as a woman who's getting abused by a couple of guys and suddenly switches from high pitch lady-like voice to "you fucking what you cunt?" In a deep manly voice.
:)