It's a pain to explain clearly, but a lot of it depends on the method of presenting an ideology, as well as the projection of that over a specific topic.

I say specific topic because there's already an amount of preconceived bias behind pretty much every sub. Political subs tend to become circlejerks, because of that very reason.

Now as far as the previously stated presentation goes. Notice how most of the top posts on Reddit are hopelessly inoffensive, and try to avoid particularly forward views? That's how a certain topic or ideology grabs a lot of attention, without turning too many eyes away. This funnels down into a ton of people with a certain mindset revolving around different subreddits.

It's all very subtle.