There is a problem with Garmin treadmill uploads to Strava whereby at the end of a run, you calibrate the distance based on the actual treadmill distance and the watch then uploads the recalibrated distance to Garmin Connect.
When synced with Strava however, Strava shows the uncalibrated distance, which can be a long way off. This seems to have been a problem for a long time, going by the age of the thread on the Garmin forum (5 years to be precise).
It would be great if the calibrated distances appeared on Strava.