Ancestry, not nationality

A while back, I saw this really cool U.S. map of dominant ancestries by county. [clicking on the image brings up the 800 pixel map with the legend.

Predictably, there is a high concentration of Italian around New York City, Germans throughout the Midwest, and Puerto Ricans in Puerto Rico.

But what I noticed when I looked again at this again recently were the counties that listed "American" as their principal ancestry. Those are the ones in yellow.

I wonder what they meant. "American" ancestry? Maybe they misunderstood that the question was about ethnicity, not nationality.

Strange.