Calorie burn discrepancy

[Kpiva]

I’m so confused. On Sunday (2 days ago), I walked 8412 steps, 5.32km, 10 zone minutes and burned 2350 calories… Monday (yesterday), I walked 10,686 steps, 7.1km, 60 zone minutes, and burned 2309 calories… so 41 less calories burnt on a day where I was much more active?? Can someone explain this to me? 
*the only difference is, on Sunday I was active for 9 hours total, and Monday I was active for 8 hours total….  But that shouldn’t equate to more calories burnt considering I was still much more active overall on Monday.

Any insight would be appreciated! 

[JuanFitbit]

@Kpiva , a warm welcome to the Community! Thank you for your message. 

Fitbit devices combine your basal metabolic rate (BMR)—the rate at which you burn calories at rest to maintain vital body functions (including breathing, blood circulation, and heartbeat)—and your activity data to estimate your calories burned. If your device tracks heart rate, your heart-rate data is also included, especially to estimate calories burned during exercise. The number you see on your Fitbit device is your total calories burned for the day.

Depending on the heart rate and the heart rate zones that you had, you might have burned more calories on Sunday, if your heart rate was higher on Sunday. 
If you tracked a walk or a hike or any other activity might influence on the amount of calories burned.