When calories are incorrect it's often because one of our profile settings for age, height, weight etc are set incorrectly. It's worth checking yours at https://www.fitbit.com/settings/profile
Another possible cause is if your fitbit is tracking too few steps that will cause the calculated calories to be too low. Do you have a feel for your step accuracy?
If the steps seem reasonable then how about the heart rate, does that seem ok? Again, if the heart rate is too low then calories will be too.
If steps and heart rate seem ok there's not really a lot you can do other than tweak your profile settings until you get numbers that you think are more accurate. Here's Fitbit's comments on accuracy:
Help article: How accurate are Fitbit trackers?
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