Whats the betting that the smk was made for markets other than the uk and they thought "sod it we'll give it a go in the uk"
Edit, I think I know how they get away with it, the valve is set at one level, the pistol barrel is machined to a length that produces 5.5 ft lbs but it wastes gas hence the silencer the rifle barrel will be able to use that extra gas to get the higher ft lbs