Possibilities –
- You need to pair your Garmin though Garmin Connect on your phone to ensure your signature is up-to-date on the device before a ride.
- The XO server removes spikes and errors while the Connect IQ field does not.
- If a BT occurred during the ride your signature is updated and results recalculated on the server.
Reference:
Garmin Connect MPA & XSS Data Field Issues - Support - Xert Community Forum (xertonline.com)