Originally Posted by Kastley85891

The air at altitude is less dense , so is less powerfull.
Any FI system will have to work harder to meet the same levels of pressure in terms of boost in relation to a sea level vehicle.
The MAF Sam refered to measures the amount of the air passing through the intake/s in a V format, that is directly related to an ECU scaled table that takes that V and performs a look up / calc to see how much air actually is going in, it uses that figure to throw in the correct amount of fuel to meet the fueling targets.
Thats it in a nutshell, but to meet fuel targets accurately there is a lot more then MAF V / air flow alone.