Fitbit Calorie Updates Seem Inaccurate

Assuming your stats are correct for both MFP and Fitbit, the adjustment should be the difference between what MFP expects you burn and what Fitbit says you have burned so far for the day. The adjustment tends to be high, as MFP assumes you will maintain your activity level until midnight, while at some point you're going to go to bed/be inactive. This correction gets 'fixed' overnight.

Example, assuming MFP expects you to burn 1800 daily (75 per hour) and your BMR is 1200 (50 per hour).

6am you wake up, and Fitbit tells MFP you've burned 300 calories so far. MFP expected 75 x 6 = 450, so negative -150.

Between 6am-9am you go for a run and do some getting ready for the day sort of errands. At 9am, Fitbit tells MFP your total calorie burn so far for the day is 800. MFP expected 9 x 75 or 675, so you're +125.

At 9pm, Fitbit says you are 1675 burned total. MFP expected 75 x 21 or 1575, so your adjustment is now +100.
*you go to bed at 9pm*

From 9pm til midnight Fitbit credits you with BMR so your end of day Fitbit calories equal 1825. When you sync the next day, and the MFP adjustment corrects to +25.
~~~~~~~~~~~~~~~~~~~~~~~~~~

So if your Fitbit adjustments are wrong, then perhaps your stats in Fitbit are incorrect and its giving you too much credit for calories burned. Such as if Fitbit has your weight wrong, or gender. If you exercise/are very active early in the morning then you're likely to see a positive adjustment early in the day, though it will decrease if you're then inactive for a spell.

Check that the time zones for both MFP and Fitbit are the same.