That's just one of those things best not bothered with. It is a measure of the caliber, but it is a shotgun. Who knows, who cares why. The mm measure is length of the overall shell. There may be some guns out there with chamber lengths designated in mms instead of inches like we use here, or possibly it's just the ammo that's marked that way because it's made elsewhere. Whatever, as long as it is correct for your gun.

You could go mental trying to figure out all that cartridge/caliber stuff.