Some of those instances are federated with yours (ie lemmy.world) but for users who are signed up on that particular instance may not have the same federations your instance has so they created their own version of the community.

Is this why I can read this while on kbin?
kbin has a 'federated' instance that's shared w/ lemmy?
i have no idea wtf I'm talking about. still trying to wrap my head around the fediverse.