Baobab, It helps to consider the engine as an air pump, for that is what it is, powered by liquid fuel.
There is an ongoing battle between the engine trying to pump air out of the inlet manifold (causing low MAP, measured after the throttle plate) and the atmosphere trying to rush in past the throttle plate to replace the air pumped out (resulting in increased MAP).
If the engine rpm increases at a constant throttle setting, there are more pumping strokes / minute so the engine wins a little and successfully reduces the MAP.
If the engine rpm decreases at the same throttle postion, the atmosphere regains a little, so the MAP increases again.