First of all, sorry if my ignorance sounds disrespectful.
I know Americans care a lot about heritage as most people are not natives. A lot of people say things like im 50% this, 25% this and 25% that. I heard people say im Irish, Italian or German when they never been to that country or speak their lenguages. But i never heard im british or english (which is odd since a lot of americans came from UK), just white, like: "im half white half mexican".
Could this possibly explain why some people in USA don't consider southern europeans or even eastern europeans as white? Because they are ethincally diferent from tipically british people.
Again sorry if this sounds offensive, im just curious.