Perhaps random isn't the right word, but it's completely out of your control, will, or knowledge. Just like your eye color.

>It is not some majestic yet universal coincidence that people have more in common with those who happen to share the same genes, and less so the more divergence exists.

It is the definition of a coincidence. The confounding variables being the shared area, culture, and socioeconomic tier in which you're born.