So in this instance, you’re posting to a group on lemmy.ml. The way groups work is that the instance the group is on boosts any posts made to that group to users who are subscribed to the group. Lemmy.ml does not have access to your block list, so boosts it based on its own federation list
Authorised fetch fixes this problem in the wider Fediverse, but Lemmy lacks a proper implementation of it at this point AFAIK