That’s a pretty crazy discrepancy. By looking at your route, with the amount of twists and turns I wonder if you need to change the settings on your watch to update location more often?
Check to see if it’s set to SMART recording- if so, change it to update every second. In the image, the SMART recoding is the top one, and you can see how this might cause an issue with a non-linear route like the one you did. This explains it more (click “Recording Method” tab):
Garmin
If that’s not it, I’m at a bit of a loss as to what could be causing that.