the maf is placed in the S curve of the tube, generally mafs are supposed to be mounted in straight tubes so air is less turbulent flowing by the sensor. with the SPT, there can be dips and spikes in the afr at random due to turbulence created by the tube bends before and after the sensor.
the inconsistency of air flow past the sensor can cause it to register higher or lower g/s measurements in split seconds, momentarily causing the ecu to account the appropriate amount of fuel for that reading(which is usually a rich dip or lean spike and you'll feel it while driving... hesitation). from what i noticed, it's almost equivalent to having a dirty maf sensor. the more you f'in know.