On SEP airplanes, why does the fuel consumption decreases with temperature, pressure and airspeed being equal?
In the examples cited, there is a slight increase in Density Altitude (DENALT), which results in a slightly higher TAS.  As the air becomes thinner, fewer molecules pass by the aircraft per mile, resulting in a lightly less loss due to drag.  Increased temperature does not have the same range of change as altitude, however, it is nonetheless a higher DENALT, which is thinner air.

As an aside, in practice, you will note that the throttle becomes wider open with less dense air, until it is fully open. (Note that RPM is not the primary measure of power, rather fuel burn is. RPMs will become higher at higher DENALT for the same power.) At that point there will be no more available power, and any increase in temperature or altitude cannot be compensated for by increasing the throttle setting.

You will also note that less energy is expended per mile at higher DENALT, not factoring in the cost of a climb to that altitude.

Finally, a different practical example of this: When flying in icing conditions, conditions (and the POH) may call for continuous carburetor heat at altitude. When this is done, the engine should be releaned, as the DENALT will be higher, and if uncompensated for, will result in a rich mixture which could foul spark plugs.