I'm no expert, but I'd hazard a guess that merging the faces of any lists of 13 random people from a given ethnic grouping will tend to create similar results to each other.
everybody knows the Doctor's real face is all tentacles and puss. He uses a psychic mask (like psychic paper) to reflect a face that is appealing or non-threatening to the viewer.
I'm pretty sure this was explained in the lost episode that I didn't see.
