JC,
The higher the temperature gets, the less dense the air which means that the engine will produce less power for a given MP. However, as you climb and the pressure decreases, you do not lose any MP or intake air density because the engine is derated. If you pull 24 inches at sea level and 24 inches at 5,000 feet at the same temps, the engine will produce more power at 5,000 feet because you have less exhaust backpressure. Therefore, to limit the horsepower to 131TO or 124MC, you can pull less MP at altitude for a given temperature.