Distance inaccuracy during running in forest (- 5%)

GPS, especially is forest, is only so accurate. Generally expectations should be to within 10%. Most of the measured, slightly long 20km loops that I've done in a few trail traces tend to be 18.5-19.5km. Others with garmin or non-garmin devices will get similar readings. Part of it is due what garmin does to make wobble within a straight run not look like wobble tends to take a bit of distance out of turns and especially from any switch backs. You'll also note that the slower you go, the more accurate the distance will be because of the additional gps points.

I don't think that there's anything likely to be done, as if the lessen the assumptions about the path that you're taking, the distance when you're actually going straight (but have gps wobble (accuracy is ~7.8 meters!) from point to point) could end up jumping much higher than the distance you lost.

5% faster than 5:00 (300 seconds) is 4:45 pace.

TLDR - not a garmin issue, rather a GPS routing issue of making the best of noisy data.

Thank you for your detailed answer. I do not fully agree though.
In fact I did this course many times with my Apple Watch (alone, without iPhone). Result: From 12,5 to 12,7 km.
Two runs with the FR645 and I get 11,8 and 12,0 km.

I think that Garmin can do something.
Or this means that the gps on this watch is less accurate than the one of the AW.

Might be best then for you to return your Garmin and use the Apple Watch. Whether you agree or not, what you see with GPS in conditions with overhead cover is what you get. Take some time to do some reading on GPS so that you can better understand the capabilities of consumer GPS.

It would also help if you would post a track of that obtained from your Apple Watch along with the one from the 645 for comparison.

I did a run with both watches this morning (Apple Watch Series 3 and Garmin 645M) :
Results:
- AW S3 : 12,4 km
- FR645M : 11,6 km
- Real value (based on computer bike) : from 12,6 to 12,7 km

The difference is quite big and makes the FR645 unusable for my daily run.
This is unfortunate since I love this watch except this (but this is a primary feature).

Both gpx here :
https://1drv.ms/u/s!Ar7_JRpF2Rt7oE2e1J21DuFDq2Fc
https://1drv.ms/u/s!Ar7_JRpF2Rt7oExF12DCMQ115BwW

Big difference of number of points recorded. Maybe the reason?

Regarding Google Maps is your track 12,49 km long. So the AW S3 is pretty close to it. And yes, looking at the 645-track, within the first 4km its sometimes really off the road. Maybe playing a little bit around with GPS only or GPS+Glonass?

Hav you changed to one second recording on FR645M?

In my experience, the Garmin is more accurate than the AW S3...

Here is a run with AW Nike+ GPS + Cellular with cellular turned off. AW on the left wrist. Forerunner 935 on the right wrist.

AW recorded with iSmoothRun.

FR 935 one second recording and Glonass turned off.

http://www.mygpsfiles.com/app/#Ht8mmx4T

The Garmin is more accurate in distance because I have run this track with other Garmin devices and the distance is always around 15,3 km.

Thank for letting me know this website. I uploaded my gpx too :
http://www.mygpsfiles.com/app/#HeuQ2WE9

Strange thing: the distance of the Apple Watch becomes 12,1 instead of 12,4 km in the activity app of the iPhone (I’m still sure that the real distance of this course is 12,6).

I use the app RunGap to export gpx from AW.

Because each software program processes the data differently.