Exactly this. It's simply selection bias. There is no filter for earning a citizenship at birth, but there is one when you immigrate.

So in America (or any nation) you will end up with a relatively average (native born) population, with disparities in earnings accounted for by the relative economic strength by historical accident of the racial/gender groups, who themselves are effectively in competition with the top 1% of foreign countries (since those are the only ones given the opportunity to migrate).

Thus you have the results seen above.