I would say this is a pretty good rule of thumb, I would bet that your Garmin is actually showing a average power of 171.8 but only shows a hole unrounded number of 171, so then Strava takes its algorithm of 171.8 * (120/116) = 177.724 and rounds to the nearest whole number 178W.
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WKO shows:
- 1:59:59 elapsed
- 1:55:06 moving
- 171.2 average power
Strava shows:
- 1:59:58 elapsed
- 1:55:54 moving
- 178 average power
My rule of thumb is to not look at those types of numbers on Strava 
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Then ignore speed/distance but keep your eyes on time/ power you want them to be close between platforms/devices but there’ll always be slight differences due to algorithms etc. FWIW, Ive had some indoor rides record bizarre speeds (200mph +) for some reason or another at least the new strava app lets you hide them from public/friend view 
This is the same example I used above, its falling into around that 3% variance zone that windwarrier and I calculated.
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