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it is your lack of exposure and chance to pair a face with a nationality.
Take a look at a bunch of pics of regular (not models or pop stars) Koreans and Japanese. After a while you'll start to notice a striking difference in the morphologies of their heads.
To me the Koreans tend to have flatter faces with eyes that bulge a bit more than Japanese.
Now I can't tell the difference between a Vietnamese and a Laotian. But neither of them look like Chinese to me.
Goes the same with central and south America. I can generally tell when Mexican is from the border, Mexico City, or the Yucatan peninsula. Guatemalans look nothing like Mexicans and Argentineans look like Argentineans to me.
At the same time I have no confidence in differentiating someone from Norway, Sweden, Germany, or England.