"I'm from China."
"Really? What part?"
"Well, all of me, of course."
[old joke alert!]
I think the skulls are probably from different locations; that's probably a problem the researcher is stuck with, since ancient skulls are not generally found nicely stratified in a single site with a long history. :')
That is what I was afraid of. In general, skulls in the south would be smaller than those in the north. In prehistoric period, people in different region could look really different.