Think i may have found another glitch, detune the power to lowest setting then slowly put it back up to 855 bhp and the pp goes higher to 796 then drops to 793 at full bhp of 905. This only happens online as well.
I totally forgot which car it was, but I remember when I was tuning a car in GT5 it was the same. At a certain point, more HP = lower PP. I wish I could remember the car, but it was exactly like this.
It could happen to the X1 in GT5. Very high performance cars can simply "break" the PP calculation. It could just be a funny thing that happens in the math when you get extreme values. This was seen during the GT5 online hacks where multiple thousand HP cars could get negative PP values.
EDIT:
I missed the part saying this is online only. That makes it stranger.
I totally forgot which car it was, but I remember when I was tuning a car in GT5 it was the same. At a certain point, more HP = lower PP. I wish I could remember the car, but it was exactly like this.
Someone may be able to debunk this theory, but my initial thought went to the power curve.
Not that you can actually see one.
But sometimes by lowering the power limiter it flattens the power curve creating a larger area from which the PP is calculated, and there's a point where the larger area, but with still with enough BHP, creates a bigger PP figure.
So you end up with more PP but less BHP than 100% power.