Another thing that struck me while I was half-way watching "The Day of the Doctor" ... if 10 used up a regeneration and kept his old face out of vanity (becoming both 10 and 11), wouldn't that have meant that there should have been 2 David Tennants circling Gallifrey when the doctor was doing his bit of timey-whimey? Never mind the fact that the whole idea of 10 being both 10 and 11 wasn't anything that was even suggested until 11 said something about it at the end of "The Time of the Doctor".
And probably a better question is why do I keep trying to make sense of genre television, even when I know that all I'm going to accomplish is giving myself a headache (especially when the show is timey-whimey)? But damn it, I want stuff to make sense.
If, someday, you guys ever hear that my brain got broken ... you'll know why. Stuff like this made it go boom.