Versa Lite calories burnt concern

[MarlenaK]

Hi I have question about how calories burnt is exactly worked out. I have versa light and I have to say it doesn't seem consistent with what I'm doing?

Just to show example 

Saturday I didn't exercise. 

Steps:10,034

Active minutes: 6

Total cals burnt for the day: 2688

Sunday again  no exercise 

Steps: 10,032

Active zone minutes: 3

Total cals burnt: 2813

How can I have burnt more calories on Sunday having less active minutes and less steps?(even if the difference is just 2 steps)

Today I didn't exercise 

It's 8pm 

I've done 9147 steps, no zone minutes and so far I'm a bit over 2200 cals burnt and I doubt I will even get to 2600 at the end of the day 

The point isn't really about the amount of calories burnt but that it seems bit inconsistent with how it calculates the numbers

Moderator edit: subject for clarity 

[JohnnyRow]

It's not like an exact formula inputting steps and minutes.  Above BMR (base metabolic rate) calorie burn, it's all based on heart rate.  Maybe one day you were doing something that got your heart rate higher while you were taking fewer steps, or took several very short steps that didn't take much energy.  Or some step were going uphill or upstairs which raise heart rate and burn more calories per step; or vice versa walking downhill.  And of course steps are not precise; you probably get a few "fake" steps using your arms without walking and lose a few pushing a shopping cart, for example.

[MarlenaK]

Right I think I understand 

So even if heart rate doesn't get into active zone minutes but is above the resting heart rate, that burns more calories too the higher the number goes? 

I thought it doesn't count it as active minutes until quite higher up because there would be very little difference in terms of calories burnt 

[JohnnyRow]

The higher the heart rate, the more calories burned.  Active zone minutes, and the different heart rate zones, in a sense, are just arbitrary cutoffs to give you a sense of how hard you are working, though the different heart rate zones might have more value for athletes in designing training programs.

But I admit I really don't the shape of the graph of calories burned vs. heart rate, if there is some point where calories burned starts increasing faster.  You could look at some of your own exercise logs calorie burn and heart rate throughout the workout to get a better idea, maybe start out slowly and gradually increase intensity, then afterward look at workout graphs to see how calorie burn varies with heart rate;  I don't mean just the whole workout totals average, but the graphs throughout the workout.