There seems to be two schools of thought on this.
The first is that there is less exhaust back pressure at altitude so the engine doesn't have to work so hard and becomes more efficient.
Therefore, to maintain a given horsepower requires less manifold pressure.
Others say the "breathing" effect is negligible and the reality is thus:
The MAP gauge is measuring the absolute pressure in the intake manifold, not a pressure differential.
Now lets say you want the MAP gauge to read 23".
At higher pressure altitudes, where the ambient pressure is less, the throttle would have to be open further to obtain 23" of pressure in the intake manifold than it would at sea level.
This results in a greater volume of air entering the cylinders per second, so therefore a greater mass of air per second, and so therefore more horsepower.
So 23" of manifold pressure at altitude generates more horsepower than 23" of manifold pressure at sea level.
In summary, an equivalent MAP reading won't result in an equivalent throttle butterfly position when you change pressure altitude. As you go higher, the throttle butterfly is open considerably more to maintain the same MAP. If the throttle butterfly is open further, the engine will generate more power.