Has the step counter been recalibrated with iPhone 7 or iOS 10?
The motion coprocessor is a different version in the iPhone 7.

Embedded within the A10 chip, there is another chip, called the motion coprocessor, M10. The new version of the coprocessor is likely to account for these errors in step counting.

In addition, this step counting is really only supposed to be an estimate. If you want an actually somewhat accurate step tracking solution, you should go for a dedicated wearable device, like a FitBit or Apple Watch. 

The motion coprocessor is simply a gimmick to match competitors' phones.