The temperature raises the density
The cooler temperature of the air above implies that, for a given pressure, it's density will be greater.
For a given power setting, 65% for example, the fuel flow should be the same at all levels, for a given RPM. However, as we climb, the temperature drops, so, for a given manifold pressure, the air density increases. The consequence is that, to mantain the same percent power with the same RPM as we climb, the MP should be reduced.
I made the emphasis at the "RPM" because, the higher the RPM for a given power, the lesser the efficiency. My protest to Cirrus Design for protecting its marvelous planes from intelligent pilots.