I'm going to tell you something that might blow your mind. Here you go:
"The calories you see on the food label or in your fitness app are not necessarily the amount of calories you absorb or bun."
Now, I hope that doesn't bring your entire world crashing down if you have been working hard to count your calories in the food you eat and the number of calories your FitBit estimates you burn during exercise. Calorie counting is really just a science of estimates. Here's why.
The calories in the food you eat are measured in a lab where measurements are based off of how much heat is produced when you literally burn something. That is a very simple explanation, but you can imagine that your digestion does not work the same as fire. SO many things can affect our digestion, such as the bacteria in our intestines, the amount of fiber in the food, hydration level, temperature of the food, and many other things. The number on the food label is actually the "potential" number of calories you may absorb.
The amount of calories we burn is even more estimated. At least the calories on the food label are a somewhat close. Let's take two different individuals who run a 5k run. They run the 5k in the exact same amount of time on the exact same course. Maybe they even where the same brand of shoes. Lots of things are the same. But what is different about these individuals is that one is a 6 foot 3, 100kg guy who is somewhat muscular. The other individual is a 5 foot 7, 65kg guy who is pretty thin. Do you think they burned the same number of calories? No? Who burned more? The taller, heavier, and more muscular guy burned more calories for sure. Why is that? The muscles mass, bone mass of this guy require more energy (calories) just to maintain his body mass, and it also takes more energy to move that body mass. This is why bone and muscle mass (lean mass) determine anyone's metabolism more than your activity. So, the energy balance equation of energy in and energy out works, but you have to take into account everything that affects that.
So, what am I trying to say here? Counting calories is wasting your time. Unless you are a professional athlete or a dietician, you probably do not need to count calories. Focus more on getting the right portions and the right foods while living an active lifestyle, and you will get the results you want while saving lots of time and money of counting calories and fancy equipment.