Thanks for the bigger pic. Much better.
I certainly understand that it seems rather whacked to have a delta peak of ~40HP (~22%) between 1st and 3rd.
Hmmm. . ..If my previous conjecture, about torque on the tires, accounted for 100% of the deltas, the 4th gear peaks would be the highest. Though there is some fairly significant noise in the top gear section, which leads me to not trust that info. Was the car bucking on the rollers?
Well. .. . here's more conjecture.
Instumentation error. Depending on the dyno, the sweet-spot range may be set up for the rollers to be spinning at say 50-90mph (surface speed). Low roller rpms could producing grossly nonlinear force measurements.
Hopefully one of the experience dyno guys can chime in here.
EDIT: What car was being tested here?