It might not necessarily be that the instances are stricter, it could also simply be that those instances are targeted more often by hate/trolls so interact with those instances more often.

It’s both of those things IMO. Being stricter comes about because of being targeted more. It’s not always just responsive reactions to abuse. Often it’s pre-emptive defederation even before abuse is received, just to ensure it never happens. And that’s the part where “stricter” comes in to it, because a lot of generalist instances are happy to respond only if and when another instance becomes a problem

At least, that’s how it works on the queer instaces I admin :)