Calories Burned

I have reached my goal! I've walked 10,000 steps. Let's see -- how many calories burned does that mean? According to my Vivosmart at this moment, that's a work out that burns 4,321 calories.

WHAT?!?!?

My weight is the same, my height is the same, I didn't mess with any settings. What happened?

I tried disconnecting and reconnecting MyFitnessPal and got the same results. I checked my settings -- I'm 200 lbs. No customized step length.

Bizarrely, the calories for yesterday and the day before have also suddenly spiked to insane heights. (4,829 calories, 5,221 calories).

Isn't the calculation for calories burned by steps taken a rather simple one? What is going on?

(Edit: by my own rough calculations, I've maybe burned 350 calories walking today.)