FitBit and MFP calories earned and burned

I am confused, I just started using the FitBit Saturday. I am very confused with it and MFP. they are different in usage of calories.
Like today and other days I can only eat 800 to 885 calories that is with what is logged as movement on FitBit..
But on MFP I am eating way to low on calories. I need around 500 more to eat..what is going on.. Which do I use.. Does anyone understand what I am asking...I sure dont..

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