Fenix 6x pro and 6s pro - different tracking data

That is a shocking difference. Do you know which is correct, or are they both off? Can you compare with a third or fourth device, like a smartphone running Strava, for example?

Regardless, your firmware may be up to date. Is that also true of the GPS firmware as well? Also, do you both use the same satellite combination and the same recording interval and power saving mode?

I can't explain it  - just suggesting things to check.

That looks like a lack of gps sync at the start and using stride length to make some calculations 

This may be due to GPS drift.

When stopping during a hike or another activity and the watch is not paused, the position error is accumulated continuously to the distance.

Garmin recommends to switch "auto pause" to on.

support.garmin.com/.../

Please have a look at the image "GPS Indoors" to see what is going on.

Do you and your girlfriend have different settings for auto pause?

I recommend to compare the traces. There must be some differences visible.

As I now see in the pic, your girlfriend has obviously auto pause=on and you don't. Therefore the pace of your girlfriend is faster than yours.

Hi all, thanks for all the replies! 

My watch tends to be the correct one (compared previously with the phone's GPS yielding similar results). It looks like hers counts less kilometers than mine. 

Both watches have auto-pause off and use gps+glonass. We both switched to GPS-only and we'll gauge results after tomorrow morning's run. We'll also recalibrate the sensors before running.

As to the pace pictured, we had stopped for the photo at different times so the calculated pace was different.

Curious of the results tomorrow, I'll report back with findings. 

Adrian s said:

Curious of the results tomorrow, I'll report back with findings. 

If you are willing to share the activities on Garmin Connect (make them public and post links to the activities) it is much easier to find a reason for this, except that one of the devices is faulty.