Why does Fitbit uses MFP calories for exercise?

[nmills]

I plan my exercise for the day in the morning on MFP, just so I have it all set out. I don't use this as my actual calorie burn as I know MFP can't be accurate as it doesn't have a measure of my heart rate during exercise.

However, Fitbit seems to ditch its own calorie counting mechanism for MFP's far less accurate one. For example, I planned a 30 minute run out today this morning - MFP predicted 300 calories burnt through that. I went for the run, it ended up being 40 minutes and my Fitbit calories went from 1212 before the run to 1983 afterward. But when I updated the app, it plugged my heart rate into the exercise it synced from MFP and kept the same 300 calorie burn and 30 minute exercise time.

I've tried disconnecting Fitbit from MFP but nothing has changed when I tried to re-sync. I know I didn't burn only 300 calories with a 40 minute, 8-minute-mile run. It's incredibly aggravating as I'm very focused on the numbers and when it doesn't sync correctly I feel as if I just ran for nothing.

[SilviaFitbit]

A warm welcome to the Community @nmills. Thanks for the details mentioned and the troubleshooting tried. 

There's going to be a calorie adjustment depending on the calories burned that you have in MyFitnessPal and Fitbit, this is why you might see a difference there. You can learn more about it by checking this article that contains more information about it: What is the Calorie Adjustment in my Exercise Diary?

Let me know how it goes. I'll be around if you need further assistance.