I wonder how they determine facial features based upon a skull. Seems rather subjective but I'm no scientist.
They layer the muscles over.
The musculature is very similar between individuals, it's the bones of the skull that determine a lot of our appearance so you can have a decent crack of what someone looks like off a skull.
That's why when people get face transplants the person that gets the new face looks more like their original self as opposed to the donors face.