How does fitbit calculate calories/activity?

Losingthedamnweight

I don't get it! If I have a day off from work and go to the park for a couple hours and get 10,000 steps, it'll tell me I can have 1700 calories. If I go to work and only get 7,000 steps, it'll say I can eat 2,100 calories. What the fuuudge?!

Is it in relation to how much of my day the steps were spread out? Cause it doesn't make too much sense to me. Getting 10,000 steps and coming home and seeing my calorie budget get lower and lower is frustrating