I think people in general have diverse hobbies, and when combined with what's usually more than a passing knowledge of a lot of their parents' careers and hobbies, you end up with a very wide well of knowledge in any sizable group.

What makes HN special in my view is that people like to share that knowledge, and this is a very welcoming place to do so. I think the average HN user may be more open to learning new things (or at a minimum learning about new things). It's a great place to share that random trivia you know with a good chance someone will be appreciative.