just like some other companies, instead of building one unit with a smaller piston and cylinder it is more feasible from their point of view to just turn the one with the bigger power down... example a 7 hp briggs is the same block, piston, etc as a 5 hp briggs. but they have different jets or exhaust to control power output. makes more sense than building an entirely different engine to have less power for a smaller application. I think the br 500 model is also a unit made just for California's strict noise and pollution emmisions.