Why are my steps sometimes jumping back when using an Apple Watch?

Apple Health automatically merges the steps of your iPhone and your Apple Watch. By doing so the overall steps can go up or down after the merge – this depends on your lead device. For the best experience, we recommend setting your Apple Watch as lead device.

Setup your lead device for sync

Open Apple Health
Go to Activity → Steps
Go to Data Sources & Access
Tap on Edit (right top corner)
In the Data Source section move Apple Watch to the top position and your iPhone to the second position.

Afterwards re-sync your Apple Watch

Open StepsApp
Go to Settings
Go to Apple Health
Tap Synchronize Again

Afterwards restart your Apple Watch and open StepsApp.