I've never banned a user but have tens of banned users on my instance... What's going on?

If a user is banned on their home instance, that ban is federated out to all instances. If a user is banned on a remote instance, they're just banned locally on that instance, and their account remains active for all other instances.

They're likely some remote users who have interacted enough with your instance to be federated over, and then banned on their home instance.