Effort vs burning calories

[wicked_sticky]

Probably not the best place to ask, but does a human you burn different amount of calories doing the same exact exercise on different days? 

I've been tracking my exercise bike workout on my Inspire HR. I pedal til the bike odometer hits 12 miles. I always use the same resistance, always stay above 13mph.
I get wildly different HR and calories burned. The FB heart rate is correct.
I expect to be some variation based on how often I sped up/slowed down, how much my blood sugar is etc, and the more sleep I get the night before the lower the calories are and the less tired I feel after spinning. But am I really burning triple the calories doing the same workout just from those factors? 

[Dominique]

It would look like the 120 calories workout is an outlier. What happens if you consider 10-15 such workouts? Would calories be all over the place, or mostly around 300?

[wicked_sticky]

Based on your reply, I'm guessing this type is not normal?

Anyway Ist post was the highest and lowest outlier. I calculated my spinning cal/min for the past 3 weeks. 
Yellow box: cycled a random amount. White box: cycled 12miles.
As far as I can tell duration doesn't matter.  Removing the big outliers is 3.5 - 6.0 
Lowest cal/min have lowest avg heart rate; 116+. Highest cal are the highest HR; upto 138. 

I did raise my max heart rate to from 180 to 200 which affects the calorie range a little. But the 20 bpm, difference is accurate. 

 

[Glenda]

I've forgotten, is the inspire worn on the wrist, or where?  Could the way you are wearing it make a difference?  When I forget to take my  Fitbit one out of my night bracelet and put it on my chest I get no steps on my treadmill. 

[LZeeW]

@Glenda   The Inspire can be worn on your wrist or on your torso, but there is a setting that needs to be changed to tell the tracker where you are wearing it.  If you press and hold the button, you get to your Quick Settings.  This is where you find the setting to go between On Wrist or On Clip.

[wicked_sticky]

The regular inspire has clip mode, mine is the Inspire HR, unofficially it will work on a clip if you don't want heart rate tracked. I specifically wear it during exercises for the heart rate.  

I want to reiterate, this isn't a problem with the fitbit,  my Inspire's heart rate matches other heart rate monitors. Fitbit does calorie calculation based on heart rate. My heart rate fluctuates doing the same work-out, I can feel the difference,the days with low calories were almost effortless to pedal

If I didn't work out at all before biking if I did other cardio immediately before the bike, if I haven't eaten in the past 10 hours before biking, if I didnt get enough sleep before biking all effect how exhausting it is to bike.
My question was am I burning significantly more calories even though the external conditions havent changed.