I'm voting with the OD of the chuck. Re. 400/500 argument - maybe the chuck cannot reliably hold the larger workpiece on the last threads of the scroll (ok, indie 4 jaw - the chuck screw), hence the manufacturer's stated limitation?

Although you are able to wind the jaws proud of the chuck body - be warned - it's a good way to lose a knuckle or two (not that a lathe doesn't present any number of ways to lose knuckles and ring fingers already)