Hi
The discrepancy in the distance results recorded by your watches may be explained by differences in how you each move your arms and/or by the availability of Location Services data.
Apple Watch uses different sensors and data sources to track the results of workouts, depending on the activity type. For more accurate tracking of outdoor workouts and to enable proper calibration, check your Location Services settings:
On your iPhone, go to: Settings > Privacy > Location Services:
- Check that Location Services (the main setting at the top) is turned on.
- In the list of apps beneath, check that Apple Watch Workout is set to While Using.
- Also enable options under System Services including Motion Calibration & Distance, Wi-Fi Networking and Mobile Network Search.
It may also help to check your personal information, on which actiivity-related estimations partly depend:
On your iPhone, in the Watch app, go to: My Watch (tab) > Health > Edit - tap and adjust items, then tap on Done.
When recording specific fitness activities, be sure to use the Workout app (or a third-party app that shares data to Health) and to choose the activity type that most closely matches your workout, including choosing Outdoor Walk for outdoor walking workouts and Other for any workouts that are neither included within nor sufficiently similar to those in the main list. This enables your watch to use the most appropriate sensors and data sources when tracking results:
During general daily wear and when using the Workout app to record workouts when Location Services data is either not relevant or not available (eg Indoor Walk / treadmill or when there is poor GPS reception), the Activity app and Workout app track your arm motion (measured by the accelerometer) when estimating results including distance and pace.
For the best results, allow the arm on which you are wearing your watch to swing naturally as you move around and during workouts (where relevant to the activity type).
If Location Services were not previously enabled, then it may help to reset your existing calibration data and then to recalibrate your Apple Watch again. More information and detailed instructions are available via the link below. Resetting your calibration data and following detailed instructions for calibrating your Apple Watch can be equally as helpful for Series 3 models and will not erase your Activity history:
Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support
For the best performance from your Apple Watch - including the most accurate readings from the heart rate sensor, which is used continually during workouts - also be sure to wear your Apple Watch snugly (but not too tightly) during general daily wear and during any workouts. Apple suggests tightening your Apple Watch band before workouts and loosening it again afterwards:
Wearing your Apple Watch - Apple Support
If the issue still persists, then I suggest that you contact Apple Support (remote diagnostics and/or mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider for assistance:
More information:
Track workouts and stay healthy with Apple Watch - Apple Support
Use the Activity app on your Apple Watch - Apple Support
