Hi all,
I find out that for outdoor rides my Garmin bike computer average power and normalized power do not match with the ones in strava, although my strava is synced to Garmin.
However, for indoor Zwift ride, the Zwift power and strava is the same.
Any idea?
Probably you should ask Strava or Garmin 
But i’m courios if the distance and avg speed is the same? Strava is computing some stuff by themself probably to minimalise the risk of people cheating to get rewards. For example the don’t use speed info from speed sensor, and calculate speed from gps stats.
PS. I don’t use Garmin but Wahoo has an option to include or exclude coasting power in avg power so maybe Garmin has similar option somewhere and it makes the diffrence?
Garmin does have that option to exclude 0s from power average so that could be the explanation. I’m pretty sure strava figures out stopped/moving time independently from Garmin which could also explain it.
For normalised power specifically, I understand that the calculations in use by Garmin and Strava differ, so should always be different. See here: https://science4performance.com/2017/04/20/strava-ride-statistics/