AFM doesn't measure preasure it measures flow. AFM measures flow of air entering the engine, now putting the AFM between the turbo and the IC, it won't be able to get an acurate reading because of the difference between high->low presure (flow) between the turbo and IC compared to the High->low presure of outside atmosphere to low presure that the engine makes.
With that setup, you are forcing air through the AFM, not pulling it through like with the normal setup.