Sync positive and negative calories.

Fitbit reports to MFP your calorie burn at several times in day, after syncs with the device, at other regular scheduled times.
But,
If you don't sync your device until late in the day like after last big meal, Fitbit is estimating what your burn is up to that point in the day, reporting that estimate to MFP.
MFP then takes that and estimates what the day will look like, and takes your deficit.

Fitbit has on option called Calorie Estimation, enabled by default.
If every day is about the same activity/exercise, and you don't sync until late at night, then leave that enabled so those calorie burns are close to right.
If that is not the case and sync through the day or the day varies, then disable that option for more accurate daily burn info.


Also, log all non-step based exercise on Fitbit to cut back on the syncs, obviously don't have to log walking or running, unless decent incline, pushing weight, or carrying extra weight - it won't know that.
Log stuff it underestimates manually - swimming should be obvious, lifting, rowing, biking, elliptical, ect. Use the Fitbit database entries.
You do need to pay attention to the start and duration time.

Do NOT log your workouts on MFP, just create a post for them as to what you did and for how long, to tell your friends about.


The calorie adjustment you see in Exercise Diary may be listed like a workout would be, but it is NOT the calorie burn of your exercise, it is the total daily difference between Fitbit daily burn and what MFP estimated you'd burn with NO exercise. It could be higher or lower than your workouts.
If non-exercise days are getting big positive adjustments - you selected the wrong activity level in MFP - increase it, so you can better plan your day and calorie allowance.


Don't create a bigger deficit thinking it's better, it's not. In fact, use a reasonable deficit, you gained it slow, try to lose it slow by wise choice, not because the body forced it slow because of unwise attempt.
This tool is designed for you start from top down diet - eating less than you burn.
Instead of bottom up, eating at some set low number, and maybe not even going up with it either if you used it wrong.