First, it's quite rude of you to call D.C. "he" now that you know she prefers "she", even if she won't pounce on you for it.Well, I surely didn't expect... that.
I really can't say that I understand why someone feels that he is really a woman and not a man. But if he really feels like that, it's his desicion and I won't blame him for it.
If you're actually interested in learning more about it, check out FTMichael's "Trans 101". The basic answer is that no one knows for sure, but it seems to have something to do with the way they were exposed to sex hormones in the womb. The result is that someone's gender identity may not match their gross anatomy. Now try to imagine how you would feel about your body if you'd accidentally gone through puberty the wrong way. I don't have personal experience with it, but from everything I hear it's no fun.
David