From the fitbit website
When you first join Fitbit, your dashboard will show an estimated number of calories burned for that day. The estimate is based on your BMR (Basal Metabolic Rate) which we calculate using the height, weight, age, and gender information you provide during sign up. If you are using the Tracker and syncing your data we replace the estimated burn with the Tracker's data plus any additional activities you might log that were not captured by the Tracker.
When nothing much is logged, we try to guess how many calories you have burned if you were sedentary. (Sedentary means you got out of bed, got dressed, went to your day job, came home, and did nothing much more than walk to your car). Once you start logging activities, we assume that you know best and we stop estimating; we use the data you provided. If you only log a small portion of your activity, we'll be missing a lot.
(If you are not using the Tracker and just using our online tools to manually log your food or activities, then you will see estimated calorie burn plus what you manually log in activities. If this applies to you, you should probably also log your general life activities - i.e. 8 hours of working at a desk or 1 hour or so of house work to get a better picture of how many calories you are actually burning.) help.fitbit.com/customer/portal/articles/1