I see karma on posts fluctuating (in particular going down) more than I would expect coming from other vote-based websites. Is downvoting really used here for posts that are not spam or trolling? Or do people just change their minds a lot?
The FAQ has: We encourage people to vote such that upvote means “I want to see more of this” and downvote means “I want to see less of this.” But I guess I’m surprised if people actually behave that way? And that some posts are controversial enough to receive active downvotes vs passive ignoring.
Yes.
What makes this surprising?
The point is to downvote content that you want to see less of, not content that you disagree with. If by "controversial" you mean "that some people don't want to see it," then I can't speak for others but I can say that personally the whole internet is full of content that I don't want to see (including and indeed especially content that I mostly agree with).
The new tagging system actually works really well for this. I set AI to have a moderate negative homepage modifier, and still get to see the top AI posts but mostly only those.