Calorie burn always less



I'm tracking my calorie burn during workouts with a fitbit tracker. I find that my total calorie burn is most of the time less than the minimum amount that Kelli & Daniel put out there for the workout. For example: Today, I did a workout that said 209-300 calorie burn and my total was only 180. I am pushing myself hard and am dripping in sweat by the time I'm done. What should I do more?