Differences in total distances between an Edge 830 and a Fenix watch...

Is autopause on or off?  Turn it off for mountain biking.

Auto pause is "OFF"... 

Is there a speed sensor on the bike. If so check that both units are using it and that the wheel size is set the same on both units.

Also check that both units are using the same data recording rate. I recommend setting it to 1 second if it is currently set to smart.

Finally if there is no speed sensor check that both units are using the same GPS settings.

You'll never get correct or consistent distance mountain biking without a speed sensor.  Speed sensors (which are really distance sensors, speed is calculated) make these things dead on accurate.

So, to clarify the GPS situation; the watch had all 3 satellite systems "ON" plus the multiple frequencies... The Edge 830 had both the GPS and Galileo systems "ON" but does not offer the multiple frequency option... Now after several more testing runs including a short run (less than 3 kilometres) and two long runs (one was 44 kilometres the other one 50 kilometres) using the car's onboard Garmin which I installed back in 2018 (can't remember the model), the car's trip computer with both the watch and Edge 830 set on the car's dash to get a good satellite reception, the results were very, very close… All 3 Garmins and the car’s trip computer were within 0.1 kilometre of each other. So now back to riding the mountain bike on the mountain, through the forest and so on… The results were very disappointing to me… On very short runs through the woods (about 4 kilometres long), the difference between the watch and the Edge was 0.7 of a kilometre, the watch always having the higher of the two readings… On longer runs (10 kilometres), the difference was 1.5 kilometres, again the watch reading higher… I have to conclude that somehow the GPS signal is not as good in the woods thus the difference but you would think for the price you pay both Garmin units should be very close in their data even when we ride in the forest… I may just get a wheel speed sensor installed on the from wheel to see which one of the two units is more accurate.

No GPS will ever provide reliable distance for MTB without at wheel speed sensor.  Buy the wheel speed sensor and you can set it up to broadcast to both the watch and Edge simultaneously, but I'm nor sure why you'd record with both.  But you can if you want.

MTB has two strikes against it for GPS distance measurement: 1) Lots of tree cover as you mention and 2) Even with one second recording you are shortcutting each corner.  MTB often has lots of these tight corners and if you momentarily lose GPS reception the unit will just draw a straight line from where you were when it last had reception and will cut off large chunks of corners.  Even on the road, a wheel speed sensor offers maximum accuracy.

All three of my bikes have their own dedicated wheel speed sensors, it's just part of getting accurate results.  Record on the Edge with the wheel speed data and have it automatically upload to Strava.  No more "Strava Tax" on your mileage.  My friend with a Wahoo and wheels speed sensor rides with me and we regularly get 20 mile mtb rides with only 1/10th mile difference.

Also, don't choose "auto calibrate" for the wheel speed sensor.  Roll it out next to a tape measurer and manually input the circumference.  This provides maximum accuracy.  I even push down a bit on the handlebars as I do it as this simulates ride load on the tire which affects circumference a small amount.

I see so I should get a wheel speed sensor, thank you... I assume Garmin makes one that works properly with the Edge?

Yes, I just ordered one online and I'll get it tomorrow.... I will calibrate it manually with the tape (saw a video of the procedure) and will only pair it with the Edge and use the "Ant+" method.... Thanks again.

Excellent.  Report back with the results, good to see some happy news on this forum at times!