Americans seem get really weird with the whole ancestry thing. There appears to be a desire to look into your family history and find something "exotic", which basically seems to mean non-English - I imagine because that's perceived as the 'default' ancestry, so-to-speak.
Honestly, who the fuck cares? What difference does it make? Nationalities aren't Skyrim races. You don't get special abilities. It makes no difference whether your ancestors were British/Irish/Spanish/French/whatever.
E: This is obviously not intended as a hateful statement, people. You have to understand that the rest of the world doesn't care about this, so we're confused when we look to the US and see them take it so seriously. We're especially puzzled when Americans say "I'm Irish" because their great great great uncle bought a pint of Guiness in the 1870s. It's an alien concept to the rest of the planet.