For one the factory rated hp was gross hp in most cases. That means it was measured at the flywheel with no accessories. Add all you accessories and drive train losses and gap difference isn't so big.
Next we need to know more info about your motor. What heads? What Cam? What transmission? What gears? Has the motor ever been rebuilt?
What was changed on the dyno? Any carb re-jetting? Any changes to the timing/advance curve? Is it still sluggish after the dyno tune.
