Remember the basic formula where power output and load produces a specific RPM.
Power Out (throttle) / Load (pitch) = RPM (for the most part). Notice that RPM is proportional to Power Out (ie increase in power = increase in RPM). Load (pitch) is inversely proportional to RPM (ie increase in load = decrease in RPM).

This means if you change just one component (power or pitch), the result will be a different RPM.
If you vary both components (power and pitch) to the same effectivity (ala power and pitch curves), the RPM will remain constant.

So, with the power and pitch curves in sync, as you increase the load (pitch), RPM should drop, however, you are adding an equivalent (hopefully) amount of power to counter the higher load. The result? ... constant RPM.

This is why you can accelerate your car. You are applying more power than resistance which increases the wheel RPM. You "think" that the throttle controls speed but it only controls power out. Physics controls the speed.