Charge 3 calories inaccurate

[HA1014]

I'm not sure if my Fitbit is making correct calculations about calories burned and was wondering if there was something I can do to calibrate it? As an example, On Thursday, I stayed home all day and my Fitbit recorded the following:

distance of 1 mile

elevation of 3 floors

exercise time of 15 minutes.

The total calories burned was 1400. 

On Friday, my Fitbit recorded:

30 minute run

distance of 2.81 miles

elevation of 9 floors.

The total calories burned was 1445.

I would think the difference in calories burned between Thursday and Friday would be somewhat higher... am I wrong? 

Moderator Edit: Clarified subject

[AlejandraFitbit]

It's great to welcome you, @HA1014.

Thanks for the information provided. 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.

Your BMR is based on the physical data you entered in to your Fitbit account (height, weight, sex, and age) and accounts for at least half the calories you burn in a day. Because your body burns calories even if you’re asleep or not moving, you see calories burned on your device when you wake up and will notice this number increase throughout the day.

let me know if you need anything else.