Sounds like one of these “up to” scams by cable ISPs. With cable internet you are sharing a connection and it is often oversubscribed.
On a 500/500 you should in theory get 500 both ways the same time, but cheap routers or *BSD based ones with singlethread networking often struggle to reach these values in a speed test as the single speed test connection overloads the single cpu core.
Gibt hoffentlich bald GNU Taler 👍