On the USDA database the entry for [this chicken thigh](https://fdc.nal.usda.gov/fdc-app.html#/food-details/1899680/nutrients), calculating the sum of its parts using the 4-9-4 rule (4 cal per gram of carb/protein and 9 cal per gram of fat) is ~20 cal off from it’s listed energy amount. A similar noticeable difference can be found on [this nutrition label for some edamame](https://www.instacart.com/store/items/item_1910923160).
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Why does this happen? I know there are tools like bomb calorimeters. Do those provide different numbers than the 4-9-4 rule? If so, is that what they use to list the total calories on the label?
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>I know there are tools like bomb calorimeters. Do those provide different numbers than the 4-9-4 rule? If so, is that what they use to list the total calories on the label?
Yes they use something different these days. Its called the modified [atwater system](https://en.wikipedia.org/wiki/Atwater_system).
Your 20kcal is likely rounding error, and between two different people maybe even more significant if one lacks the needed gut bacteria to actually digest it(novel meal vs regularly eaten meal).
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