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.)
Are those downvotes maybe coming from non-Lemmy instances?
As an admin you should be able to see the downvotes of the post that made it to your instance.
I'm wondering if some software might be broadcasting votes to all linked instances, while I believe Lemmy only sends them to the community instance and it's the community instance's responsibility to relay them.
That's exactly how I thought it was. But when I checked it was ~15 downvotes across 4 different instances, and checking other posts leads to the same theory: That we are ingesting downvotes and federating them out, but not displaying them. (I'm checking a backup later today)