444HP???

I strapped my truck to the rollers yesterday and I only made 444 HP. i made 20psi less boost and could not get the egts above 1200*. The dyno was an inertia dyno. It had zero load and i still managed to smoke my tires on the rollers. A Pulling truck there with a 13mm pump, 5x20s and a s472 only made 480hp. He also hit 160 mph in the truck. So what's the deal? Was the numbers so low because of the dyno being a no-load type??