FitBit run tracker reporting significantly lower calorie burn than Map My Fitness, Endemondo etc...

Hi all, I’ve recently been using the new run tracker function of the FitBit app, but it’s giving me much smaller numbers in terms of calorie burn than everything that I’ve previously used.
10k on Fitbit seems to give me around 550-600 calories, where as Map My Fitness, Endemondo etc… all seem to say around 1100.
How do I know which to believe?

I’m 15 stone, male and 6 foot tall if that helps.