Remember that the 147 2.0TS comes with standard VDC which intervenes and cuts engine power on the rolling road. I bet that your power curves are jagged over 4000rpm, right? This is the VDC cutting in. Same goes for GT 2.0 JTS that also seems to under-perform on the rolling road but shows its 165HP on the road.

You would need a special kind of rolling road capable of spinning the rear wheels to match the speed of the front ones so that the VDC does not think that the car is slipping and intervenes.