When using your iPhone combined with your Apple Watch, both devices count steps separately. 
The steps from the iPhone and the steps from the Watch are both saved in Apple Health and Apple Health tries to combine the steps depending on your lead device. 
After merging the steps from both devices, the step count can go up or down.

To see the steps merged, you have to enable the Apple Health Sync.

Apple Health's data usually updates every other minute. In this way StepsApp on your iPhone is 100% in sync with Apple Health, therefore your step count will not update immediately. On the other hand, the step count on your Apple Watch is always live.

Short Example:
​Watch: 100 steps from 10:55-11:00pm
iPhone: 150 steps from 10:55-11:00pm
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If the Watch is the lead device because is the device you use the most, Apple Health will take into account the 100 steps and those will be displayed in the main app as well.
At the end of the day StepsApp and Apple Health should have the same steps.

Please setup your lead device for sync:

• Open Apple Health
• Tap on "Activity"
• Tap on "Steps"
• Tap on "Data Sources & Access"
• Tap on Edit (right top corner)

By clicking on the three lines, move the Apple Watch cell by data sources into the top place. Second should be your iPhone (or the device you use the most).

After that, you may need to resync your Apple Watch: 

• Open StepsApp
• Go to "Settings"
• Tap on "Apple Health"
• Tap on Re-sync again

Afterwards restart your Apple Watch: https://support.apple.com/ and open StepsApp.