It seems that this happens when you subscribe to a community on a different instance from your own. You can still read, post and interact exactly as if you were fully subscribed, you’re just not counted in the number of subscribers. I’ve been told that you can force the system to subscribe you properly by repeatedly unsubbing and re-subbing, but I’ve had no luck with doing that so I just leave them alone now.
I think I’ll remain agnostic on that one. Ask me again in 50 years and I’ll probably know the answer by then. Unless I happen to somehow reach the age of 106 without dying, in which case I’ll take a raincheck.