So that you can't trace the true owner and harrass them or if the car is at a dealers, contact the owner direct cutting the dealer out of the bargain?

Maybe they haven't verified if the car is stolen yet? (in a car yard)

A Dealership might be issued with a few sets of specific plates to use whilst they have the car, or if it is oput of rego kind of thing, like a temporary rego.

Over here in aus, lots of cars that don't have their real rego as they've run out/been in the car yard for a while, the dealers have special plates they can use for test drives, so the car is sort of registered iykwim.

same for new cars that have never been sold yet.