May be it's just me that has this problem, but just in case you do too....
I got two power meters Rotor Power and Powertap (hub) and naturally they don't really agree on all the numbers, which is understandable: different points of power measurements, PT is old I didn't bother to calibrate, etc.
I know PT reads a little lower, I can do the math... sometimes... until the brain gets oxygen deprivation from an interval. :)
So I wrote a super simple Data Field for Garmin Connect IQ to compensate the difference. You have an option to set a multiplier or get scientific and set a nonlinear regression. You can say "100:105,200:220,300:350,600:680" that'd mean when PT shows 100w it should be 105, when its 300 this should be 350w and so on, there is linear interpolation to spare your battery. You can also set an arbitrary averaging interval. And/or, with a multiplier like 1/FTP*100 you can average your power in percents of your FTP as you like. Both settings, if valid, always apply.
The App does not modify original data which goes to FIT file. There are tools for that.