Just to add: Another reason companies underrate their cars' horsepower ratings is to make higher models look better or to leave room open for quick power boosts in coming years. A good example is the Camaro Z28 and the Corvette C5. The Z28 is rated at 305hp and the Corvette at 350hp. The Z28 actually dyno's at about 305hp to the rear wheels, meaning that it actually makes closer to the 350hp that the Corvette supposedly makes. It just makes the Corvette look better, can't have people paying $50k for a car that performs the same as the $25k Camaro
