FitBit and Excersize calorie count

Does anyone know if the fitbit gives an accurate calorie count for excersize? I ask because I worked out for 28 minutes and looked at the calories on the fitbit and it said 600 burned. MFP says to enter the start time which I did and that it would automatically sync with the fitbit..but it only gave me credit for 20 minutes and 200 calories? Am I doing something wrong?