Fitbit calories burned not adding up

[gefiltefishee]

Ok to preface this question, I've read MULTIPLE posts regarding this and none seem to quite answer my question.

I fully understand that the total calories burned includes my BMR. I get that. I understand the stepped calorie reports throughout the day. I fully understand that.

Here's what I don't get:

My BMR, from what I've been able to calculate on other sites (Fitbit doesn't seem to let me view what it's calculated as my BMR) is around 3,100 calories - just for existing at rest.

The only exercise I'm doing is regular walking at my job, and walking my dog. I'm getting roughly 9k-10k steps a day. Today I got 9.5k steps. 

Fitbit claimed my total calorie burn for the day was 6,592. If you subtract by BMR from that, this means Fitbit is saying my 9.5k steps earned me an extra 2,908 calories. How is that possible? Even including my BMR, I have no idea how Fitbit arrived at 6,592 calories burned for the day, unless my BMR is significantly higher than I thought - but I doubt it's 3,000 calories higher than my estimates elsewhere. 

Now, for a good chunk of my day, my heart rate is 120+, which Fitbit says is in the fat burn zone. Could this be what's causing so many extra calories to show up? Because by what I'm seeing, I could theoretically eat 5k calories a day and still lose weight. I know that's not right.

Can anyone shed some real light on this? 

[JohnnyRow]

Can't explain it all, but maybe some insights.

Your BMR according to Fitbit:  you can get very close by looking at calorie burn chart lowest value overnight when you are asleep.  It shows in calories per 15 minutes so multiply that by 96 (?) to get daily BMR calorie burn.

Calorie burn is calculated from heart rate, not step count.  Steps are a factor in that they tend to raise your heart rate.  But if you were to get your heart rate to the same level somehow without getting steps, that should show same calorie burn.

My question would be how accurate your Fitbit-measured heart rate actually is, being in fat burn zone much of day.

And of course your weight is a very big factor.  I don't expect people to post their weight in public forum but for 3100 calorie BMR, I expect you are a big guy.