So I did a 20 min lap effort and my avg power for the lap was 269. I get home and TR, Strava and GC don’t recognise it as a 20min pb (last was 268).
Strava had the effort at 255, Garmin didn’t even register it close to my best effort.
Why the difference in numbers?
How are you comparing this? If you look at the pdc what does it show you?
Just from what my avg lap power is.
Sorry for ignorance, but what’s PBC?
PDC == power duration curve. Basically your best power at every duration from 1 second out to 3+ hours
PDC is what @AlphaDogCycling described. If you can check that on Golden Cheetah and Strava it should show discrepancies in your 20m power.
PDC is 268 all time but today on the 20min lap it was 269 however this did not record as the new all time high.
It could be a discrepancy in the way the head unit counts on the fly and the way software analyzes the actual data.
Do you have WKO or golden cheetah to look at the file with?
Anecdotally I have the same issues. I have Garmin pedals (XC and road) some Faveros, a Quarq and then my smart trainer. They’re all different 
so I have to take it with a pinch of salt and kindve used RPE and HR but yeah it’s annoying and life would probably be easier if I just used HR / RPE tbh.
I feel the pain, just makes it hard to go for long range power targets if my display on my bike computer isn’t what is being recorded on my training log (GC, intervals.icu, etc)
You are talking about a few watts. It’s not going to affect your training one bit. Your best bet is just go by one analysis software and not worry about strava high five badges.
I had a thought - did you set up your Garmin/power meter with “smart recording”. I seem to recall that that is the default on Garmins. I have a Stages and had to do some gymnastics with the setup. I had to make sure it was on Ant+ (not bluetooth) and set to every 1 second recording instead of Smart Recording.
If it is a lap… just try to ride it faster than you did before?
Is it only a single w difference? (268 vs 269) or larger?
If you go look at the ride you did the 20m effort in on TR and compare the PDC to another platform like GC is the PDC different?
I would assume that the average power figures should align on all of the different platforms, but Normalized Power or Weighted Average Power, etc., are all different, so those figures will likely vary from one to the next.
If you’re only recording power with one device, I can’t see how the averages could be shown as different from one platform to another.. 
Can you confirm that the lap data (time start & time end) are the same across all platforms?
Also, could you show some screenshots of what you’re seeing?
Two things that could make average power “legitimately” differ across platforms:
The first one is “easier” to make consistent