I've had the Element Bolt for just over 4 years now and have noticed similar differences in what is displayed on the Bolt when I finish the ride vs. what is shown when uploaded to Strava. I don't think the difference has ever been more than 0.2 mph, which is pretty small, in fact 1% of 20 mph. 1% seems like a reasonable level of accuracy as that is what a typical power meter would give you as well. My guess is that it's either a rounding difference somewhere or due to more accurate elevation and/or route information that Strava has gathered. GPS is very good, but sometimes it misses a datapoint or two which could be added back in with some sort of path smoothing algorithm. I should add, I think I remember having the same discrepancy with my Garmin Edge 500.