"How is this possible?"

Let's pretend m.s is connected to 30,000 instances. The example user has 1000 followers across 100 instances. Making the post sends it to those 100 instances.

But when you delete it doesn't know who could possibly have a copy of the post, so it sends the delete to all 30,000 instances. Even if that instance has never seen the user before. It can't be certain, so it has to send it everywhere to be safe.

Gigantic waste of resources