The latter is the easiest explanation, it has never been guaranteed that registry numbers would pass from one named ship to the next (in fact we've only seen it a couple of other times). Especially as the NCC-1031 was classified historically, odds are they would just roll out the next registry on the list the next time they made a USS Discovery.On the Registry.. So there was NO U.S.S. Discovery ( a fairly common name for a ship) in 900 years?
Don't have to have the same registry, so a NCC-345456 Discovery didn't exist in say 24th century?