Fitbit Calories vs hrm

helcart01

I have just completed half an hour on a treadmill doing week 2 of C25K. I always wear a hrm (Polar with chest strap) when I exercise. Just out of interest I also pressed the button on my fitbit ultra to log the activity. According to the fitbit I burned 256 calories while my hrm said 207 (a difference of 23%). Which is most likely to be correct?

(Also posted on fitbit users group)