Although this is a contradiction to our normal knowledge, it is easy to explain: If we want to drive a car that has 50HP at 150km/hour, we open the throttle valve through the foot accelerator, for example, in 40%. If the engine performance is increased to 110HP, we need to adjust the throttle valve at 37% only in order to drive at 150km/hour. With this changed throttle valve adjustment there is less fuel drawn in, that leads to a reduction in the consumption. In the practice it looks a bit more complex, since we cannot work with the engine with any less fuel or any more air. Here we have the secret of the right mixture, that changes from engine to engine, and can be different even at identical cars.