This is NOT it, not the reason. The reason is described by Superlyte27: The fact you need to fight the "up" force of air acting on the 2nd stage diaphragm. You basically need to pull it "down" ward to activate the orifice. Same reason you get freeflow when mouth piece is facing up and no water in the 2nd stage.
You are saying the same thing. The up force is the differential pressure.