So I think I know where those excessively high burned calories are coming from

[Jimmu]

My Fitbit often shows insanely high calories (frequently around 5000 cals day) that's half marathon level of exercise which I am not even close to doing. I sit on my **ahem** all day during work, I enjoy playing video games at night and once in while I run for 30 min or lift a little weight.

It is somewhat of a known issue and I think I know now where the problem is coming from.

I am by no means an expert but here is what I figured out after a few hours on the internet. Total caloric daily expenditure is made off BMR + NEAT (Non Exercise Active Thermogenesis) + Exercise + thermic effect of food. Food based energy is measured in calories (kcal) a scientific unit measured by literally burning the food and measuring the energy that is released.

When your body uses energy it does so through the Krebs - Cycle, basically the same process like burning (oxidation) just way slowed so we don't burst into flames.
So far so good
We measure caloric expenditure by measuring CO2 atoms exhaled during exercise.
Here is the crux.
This is the total caloric expenditure: BMR + NEAT (Non Exercise Active Thermogenesis) + Exercise + thermic effect of food measured.
So when I run to the fridge because of munchies or take a particular strenuous **ahem** I burn an additional 5, 10, whatever calories but Fitbit adds those + the BMR because that's included in formulas in the scientific literature + the base BMR that's always there.
Long story short: Essentially whenever just doing the mildest exercise Fitbit counts BMR double this adds up over the course of the day and how we get those wrong numbers.

I might be wrong but it just makes sense (I crunched the numbers on a couple days and it seemed to check out)

[JohnnyRow]

My take on it:

When we have heart rate available, that takes into account the extra calorie burn sources you mention.  Calorie burn can be considered just as BMR (basic metabolic rate) + calories from exercise that raises heart rate.

When you have a day with Fitbit showing extra high calorie burn, take a close look at you heart rate chart - I find it easier to see detail on fitbit.com dashboard.  Calculated calorie burn is based on heart rate.  I suspect on those days, you will find periods with unreasonably high heart rate, perhaps your Fitbit reading heart rate incorrectly (yes, that can happen), thereby overestimating calorie burn.  Or first look at daily calorie burn chart in 15 minute intervals;  when you see a period with extra high calorie burn, look at your heart rate during same period and see if it seems reasonable.

[Jimmu]

I have actually a relatively low resting heart rate (55-57 on most days) so it shouldn't kick in all the time. It's also a problem that persists throughout all the fitbits I had which are many. It is possible that all read wrong data but than we have a much bigger problem. Its also not a one time thing it's consistently at least 1-2 times a week I "burn" more than 5000cals that's just improbably given my life style.

It also adds up when I compare exercise data when it reads much more often. I do an hour of weight and I burn 700-800 cals if I manually calculate my projected caloric expenditure on any site its about 300-400.

I would be curious if that's the case with you too. Take any random exercise google a calorie burn calculator type in your stats and compare. My guess is you will get the same results.