The engine runs with excess air, which means the engine will draw in an amount of air that is equal to the displacement of the cylinder on the induction stroke.
The volume of fuel injected is dependent on the torque requirement of the engine and will therefore alter with load but the volume of air will not, it will only change due to boost pressure (forced induction)
So why bother to fit a AMM if the air flow could be worked out via the cc of the engine and the current boost pressure?
Modern cars, Its all wizardry and witchcraft