I'm sorry I'll try that again;
When I pull carby heat the intake air is thinner so the engine slows (or the govenor opens the throttle) causing a rise in MP.
When I climb the intake air is also thinner which should tend to raise the MP. But other factors (less atmospheric pressure on the exhaust, blades easier to turn) overcome this trend leading to a net fall in MP.
Its interesting isn't it when there are competing trends how one wins out. Like when you climb the air gets thinner, its gets colder too which should make it more dense, but the loss of molecules is greater than the decline in their speeds which means ultimately less density.