There are lots of variables that make it hard to dial things in very closely. The weight of the car changes through the race as you burn off fuel, for example. Drafting, wind conditions, wether the track is level, etc all affect the numbers that the TM puts out. Then there are the adjustments for rolling resistance, aero, etc. There is a set point in the TM for the frontal area & CD of a 944, but I doubt this calculation takes in to account the open windows we run. Then the 924S is slightly different again. These make for a lot more variables than putting a car on the dyno, and more variation in the reported HP output, from what I've seen on my own TM, and the other data I've seen personally.
Steve, were the numbers consistant from lap to lap on your car?