Probably not helpful for you but I had similar trouble between my kickr core power and a single sided 4iiii power meter.
I tried a bunch of tests, dial recording and editing the offset on the 4iiii. Had anything ranging from down 11% to 2% variation.
At the end of the day i settled on “consistency” so use the 4iiii for power via the kickr control with ant+. i’m then using the same measurement source indoor and out. I then have another 4iiii on another bike which i figure is closer between two 4iii left side PMs than between a single side and kickr.
Think my left leg is a bit weaker from a previous broken ankle