Hi Dave

The Key word you used is "calculated".

The ECC V system determines the air charge / Mass / Density, going into the engine from a measured value of mass airflow. It compares this value with a predicted value of air mass going into the engine,this is the value the PCM infers. Differences between the two values are first attributed to inlet air temperature, which is measured, and then to a change in barometric pressure, which then becomes the inferred barometric pressure.

So basically the PCM as stored Tables and "3D MAPs" which is cross checks the The measured MAF readings with the desired readings.

So if the Air flow readings are lower than predicted at lets say Idle , half or full throttle then it infers the car must be at high altitude and thus a lower Barometric pressure.

The higher you go the the air pressure get less , thinner and also less dense.