Calorie adjustments for exercise?

The only exercise I do is walk but was wondering about calorie adjustments made.
I use my I Phone to count my steps. When I use that map my walk ap it adjusts my calorie count in MFP after I go for a walk. When I don't go for a walk my exercise calories are also adjusted on MFP with the steps I so through out the day.
Here is the dumb question now. Why when I use map my walk,and it adjusts my exorcise calories, doesn't MFP adjust exorcise calories for all the other steps I take?
When I use map my walk the only excercise I get credit for are those steps and no others.
I have it set to sedentary.
I don't eat back excercise calories but was just curious why it doesn't account for all steps.
Thank you for all your help