You have the option to sync your activity data with Apple Health, this is specifically important when using an Apple Watch or a Third Party Watch to make sure your steps are precisely counted and merged together.

Sometimes it can happen that the access to Apple Health is no longer granted when downloading the app for the first time or when changing to a new device. For this reason is very important to check the Apple Health settings in order to ensure accuracy of step count.

Check the Apple Health settings by following these steps:

1. Open Settings 
2. Scroll down and go to "Privacy" > "Health" > "StepsApp"
3. Enable all switches
4. Start StepsApp