help with fitbit

I'm so confused. I got a fitbit last Friday and have it linked up to MFP. I still track my workout on here and it looks like my food and workouts transfer over to FB. I see adjustments made on MFP from FB and my question are:

1. How are the adjustment calculated?
2. Am I double dipping by putting in my workouts here?
3. How can I confirm that FB is doing the actual and not estimating my calories burn?

Thanks and I appreciate any help anyone can give me.