Calories count and burned

The active calories in your activity tracker are "real" active calories, excluding BMR calories (resting calories). This is the number you want to look at.

Opposed to that, the calories in a logged activity will include your BMR calories. This makes calories in an activity more or less useless unless you always train so hard that BMR becomes insignificant. If not, you will need to subtract your BMR from those calories to find your real active calories.

Example:
If we assume that your BMR is around 80 kcal/hour (1920 kcal/day), you spent 57 BMR calories in your first 3,74 km trip and 35 BMR calories in your second trip. This leaves 245 "real" active calories in your first trip and 254 in your second trip. So your second trip which included running actually did burn more active calories.

Ok, that makes sense now. It is more clear now. But only one thing - does it use HR for calculating calories burn or some other algorithm ?

Probably some combination of HR, steps and other accelerometer events.

It is probably not the same in every Garmin watch, but here is what I have seen in my two Fenix watches, one with a wrist HR, and one without:

• Calories from daily steps seems to be closely related to the step count. Approx. 30 kcal per 1000 steps.
  
• Calories from logged activities seems to be closely related to my heart rate. All activities I do (walking, running, biking, indoor rowing and sea kayaking) will have an almost identical relationship between kcal/hour and heart rate.
  
• Calories from non-logged, non-walking activities seem to be something inbetween. For example, if I take a bike trip without starting a logged activity, I will get some Active Calories in the daily calorie tracker, but not as many as if I had started a logged activity.