Think I've found the issue.
The watch gps is not sampling often enough and is cutting corners etc which is why it is reporting short on standalone.
Connect to the phone it works exactly the same as my garmin.
Annoying!!
Are your distances the same in s health as when you export to Strava?
S Health keeps under reporting distances compared to my Garmin 235. But when I export to Strava its a lot nearer the Garmin??