GoKawiilThe 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. Providing an Apple Watch or other wearable with your height and weight can help make calorie counts more accurate, yet ultimately, there are too many variables at play to offer truly accurate results.
A more recent study (published by Nature) also found the Apple Watch and similar wearables couldn't be relied upon to provide accurate calorie expenditure results. According to Nature's findings, the fact that smart watches use multiple sensors to estimate burnt calories is problematic, because "errors from individual inputs may compound." The site's study also concluded that factors such as a wearer's skin tone or moisture levels also impact a wearable's ability to measure energy expenditure.