[hpkuarg]

It's more possible than you think for people who share the same ethnic background to have had radically different experiences from each other, and people who look very different from each other to have gone through very similar life stories.

I'm not just saying this in theory (which is trivially true) -- I lived in a stereotypically liberal cosmopolitan city where most people I encountered, despite the visible diversity in their ethnic backgrounds, all had four-year degrees, worked in high-paying knowledge-based jobs, and had no experience in military service or single motherhood; then in a much smaller city in a stereotypically conservative rural state where most people were of the same ethnic background, but had more variety in what they went through in life. My own habits did not change between the two cities as to put me in touch with such a different group of people between them.

This is just anecdata, but I question the premise you appear to take for granted.