In reply to Newbuild100:
It might be that the Garmin is on Autotrack, power save mode. That takes a GPS reading every few mins, not every second. If your route had lots of zig zags, it will straighten them out, so it will show as less. Not sure why there would be a descrepancy between the device and the app.
Top tip, if your time is out in Strava, record it as a race. Strava usually strips out any stops, recording only moving time. Race mode is from start to finish.