Have you never seen one of those shows like "Who Do You Think You Are?" and such.
In it, it basically takes someone who swears black and blue they are "English, through and through" or whatever ethnicity, and after conducting genealogical tests, often show that the person who considered him or herself "(a particular ethnicity) through and through" are in reality descended from a number of different places/peoples.
My point is that ethnography can be a tricky thing, and just because a certain name has certain regional roots, does not definitely mean that people named as such did not migrate and move around the country/world at times.
Mickey i'm not sure if that show your referring to is what i'm thinking of but i remember a couple years ago a bunch of celebs gave up samples of their DNA to be tested and one of them was Larry David. His results came back that he was 37% American Indian. When they told him he thought they were just joking around but they weren't and according to the tests He is 37% American Indian. He thought or assumed he was 100% european.
http://www.huffingtonpost.com/megan-smolenyak-smolenyak/playing-with-dna-is-larry_b_402795.html 
