[Christ]

Because the engine hadn't begun putting out any more power as a result of the part installation. The"net gain"you're referring to isn't a gain, its a reduction in losses through a system of components.

The only factual gain you could truly claim is an efficiency gain because by reducing the rotating mass, you've made the engine work less to accelerate it to the same speed, and by slowing down the accessories that it drives, you've brought the belt drive closer to 1:1, making the interface ratio more efficient, meaning less hp is used. You still haven't made any more hp from the engine though.