fitbit zip and hrm

When you log exercise on MFP and have FitBit synced, it asks for the time and duration of exercise.
That allows replacing whatever FitBit estimated for that time.

So if they agreed, there would be no change to FitBit.

What you are seeing in Exercise diary are adjustments, based on comparing what FitBit says you burned for the day. It may or may not have actually been exercise, but that's the way to get your maintenance to go up to match FitBit.

Now, outside of that, MFP is going to received what FitBit says you burned in total for the day, exercise included if done, whether that is FitBit estimated or corrected from MFP.

If that value is higher than what MFP has down as your non-exercise maintenance, you get a positive adjustment.

It means you burned more than what MFP thought. That value then has your deficit taken off, and your eating goal just went up.

Now, if you logged the exercise in MFP, it already knows about part of that positive adjustment, so the adjustment is lowered by the same amount.

So you can see this on one day in the past. See how many calories FitBit said you burned for a workout you never entered in MFP.
Notice what the adjustment is that came across, and your eating goal.

Now log a workout in MFP of exactly the same calorie burn and time and span, and sync.
Your adjustment should have gone down by equal amount, eating goal stayed the same.

Now edit that workout to be more calories, say it was exercise FitBit no good at estimating, so trust the Polar more.
Adjustment should stay the same, but your workout just increased the maintenance figure, so your eating goal went up.

Now add to that process which is usually clear when looking at the whole day, is that through the day MFP compare's too and if it looks like your daily maintenance is going to be higher, it starts throwing in adjustments as the day goes by. Auto correctly each time you sync.
So if you did a morning workout but didn't log it in MFP, it might just appear you are going to have a big day burn, so big adjustment, even though it was only 1 hr workout, rest of the day will be normal sedentary.
By the end of the day MFP has that figured out and adjustment goes away. But you may have eaten differently through the day.
But if you log that workout soon after doing it, MFP knows it was exercise, and adjusts better.