That's why I run with phone in my pocket. It adds a km per 5 km standalone, and both in Endomondo and Samsung Health.
However, is it the same for LTE models?
I have an inkling that if there is no Internet to get map to match GPS it just uses step count to measure distance. Would be quite easy to test. Walk the same distance with GPS on and then off and compare, but at least few kms for better spread.
I thought I was alone.
Watch (Gear S3 classic) says I ran 18.5km when I ran 15km. I didn't notice it was off till last week's Sunday run. All the while I just thought I was getting faster.
But last week Monday, it recorded my 5km run accurately. I cross check all my runs now with separate timer and Google Maps.
I run WITHOUT my phone. And I always wait for my GPS signal lock before I start.
Just hoping there's a fix.