evo north

A scooby went on some dd rollers today, was peaking at 1.6 bar, but started to loose boost at around 4000rpm, ended up at only 1.0 bar at the end, power was only 262bhp atw, should of made 280 bhp, but the car doesn't feel like that on the road, seems to pull very well to 8000 rpm, the car is very fast aswell, can the rollers be out, how are you guys working out the atf from the atw fiqure please