Just how? Would that just be Garmin vs Polar?
You don't have a barometer in your watch so Strava *should* over-write the elevation over what Garmin "guesses" it to be from its own, slightly different, elevation dataset. I've stripped out the data Strava took from Polar (675m on Polar, becomes 749m on Strava) - and using Strava dataset now gives me 656m.
If you get the option to reset your elevation to Strava may be interesting to see what it gives.