This reminds me of one party in UC Berkeley hosted by my extremely Democrat-voting, racism-fighting friends. All their friends and acquaintances were there. 20 minutes in, I noticed something was different... every single person out of ~100 attending was white. This is statistically unlikely, given that >50% of the students are (or were?) Asian. And I'd been to plenty of other parties not like that.

Then I realized, that entire friend group is distinct from my other groups, with almost no overlap.