Why are calories burned inconsistent if Exercise is not activated?

[Windows Central Question]

Calories burn inconsistent if Exercise is not "activated"

I tried to check band performance in 2 conditions - Exercise mode "activated" and exercise mode "not activated"

I played tennis for 1.5 hrs in both cases. When "activated" it shows my calorie burn in the range of 1000 with heart rate between 130-180. Whereas if it is "not activated", heart rate remains in the range of 80s and calorie burn around 300-400.

Is this a problem with band or regular feature that i have to activate the exercise mode?

Thanks,
Mayank