Difference between Active Calories and total calories?

Ok I think I figured out the difference between active calories and total calories.? Let me know if I am correct here and what I see wrong with it....

So when you finish an exercise you have active calories and total calories. If you were to subtract active calories from total calories the difference would be how many calories you would have if you did nothing in that amount of time you used for exercise?

If thats the case what am I going off of when considering the calories I burned it that activity? My old garmin if I went for a run It would just say I burned lets say 500 calories. So is the garmin giving total or active?

And sometimes there can almost be 50-100 calorie difference between my active and total depending on how long the exercise. So apple is trying to tell me if I sat and did nothing for 1 hour instead of a bike ride or run that I would have burned 50-100 calories? I dont think so. We would all be skinny if we could average 100 calories burned an hour doing absolutely nothing.