Hey juliette1980,
We understand there's a mismatch of data between your Apple Watch built-in apps and the 3rd party apps installed from the App Store. While we can't specifically offer guidance from the 3rd party app, we can suggest Calibrating your Apple Watch for improved Workout and Activity accuracy.
Always ensure your Watch and connected iPhone are up to date in Settings > General > Software Update:
Update your Apple Watch
Update your iPhone, iPad, or iPod touch - Remember the importance of backing up first (How to back up your iPhone, iPad, and iPod touch)
Then check for app updates of the Strava app you have installed, maybe considering remove and reinstall of the app if the data will be preserved for you.
How to manually update apps on your Apple device
Thanks for posting in the Apple Support Communities forum.
Enjoy your week!