The downvotes you can see (on this post) are from accounts on your instance then. As this post is semi inflammatory it is highly likely to have garnered some downvotes.
Edit: I guess I was wrong regarding the logic of how downvotes work when we block them. As the http request (used too?) return an error when responding to a downvote. I'll have to look at it again. As the only way it was/is 15, is if:
we kept track of downvotes and sent out the activities notification
your instance got the notifications from other instances about our post, (which is not how Lemmy works unless I'm seriously misunderstanding it.)