Your Apple Watch calorie tracker is an educated guess, not a fact

The easy answer is that truly accurate calorie expenditure can't be obtained from your wrist. As smartwatches are unable to measure your metabolism, they have to analyze data such as heart rate measurements or the distance you've covered during a run before they can make a ballpark guess at how many calories you've burned. Whereas fitness wearables can directly measure your heart rate from your wrist, energy expenditure has to be calculated through proxy methods, such as machine learning algorithms. Anna Shcherbina, an assistant professor who also worked on the Stanford University study mentioned above, partially blames the algorithms wearables are using to assess calorie output. "My take on this is that it's very hard to train an algorithm that would be accurate across a wide variety of people because energy expenditure is variable based on someone's fitness level, height and weight, etc," stated Shcherbina.