Short answer: Yes.
Longer answer: As someone who rides with a PT, I don't normally look at or think about the KK power curve *while* I'm riding. Recent rides without my PT have made me look into this more closely, as there seemed to be a noticeable discrepancy between the two. A torque test of my PT has led me to believe that the PT is at fault (~7% low), and the KK power curve would have matched within 1-2% the readings with this fault taken into account. I've started going back through my ride files to try to find when my PT may have gone out of calibration, and here's what I've come up with so far:
Disclaimer: this is just a first pass for me, and I'm not offering this up as a scientific test. These were just avg power and avg speed pulled from some intervals to see if I could identify a
point in time where a major shift occurred in the PT. Since I ride with a PT, I don't do coastdowns, monitor tire pressure, or take any steps to ensure a consistent *speed* between rides, and that's why there's such variation between the points. As another poster has already pointed out, averaging power over an interval of varying speeds adds it's own issues of non-linearity, but I've not had a chance to go back and adjust for that. At the high-end, those yellow points are ~7-8% below the KK curve, which matched the error on my torque test very closely.
See short answer for less details.