AFM measures the volume of air going into the engine, a map sensor guesses the volume by comparing revs, pressure, and throttle position.
The volume of air is what realy counts, as it's the amount of air getting into the engine, not what pressure is behind it that gives you power. and say you hit full boost at ~ 4000rpm, the map sensor is going to read the same manifold pressure right till redline, and will have to guess the load axis for the ecu.
i have no idea if that makes any sence. But do a search, it has been covered heaps.