Does a fitbit over estimate the amount of calories burnt?

How accurate is a fitbit for recording home exercise in front of a DVD? I've been wearing my fitbit for walking and the calories burnt seem sensible for my duration etc. However, I just did 20 mins of a high Intensity 'pump it up' DVD and it is saying that burnt 300 calories. It was 20 mins without stopping of star jumps, jogging on the spot, bunny jumps, kicks etc so intense bu 300 seems a lot for 20 mins of exercise.

I just wondered what people's thoughts are on how much to trust the 'calories burnt' in terms of logging and then eating?

Thank you