This is what I don't get with regards to calorie labels. As far as I can tell they either burn stuff to see how much energy gets released or count up macro nut

This is what I don't get with regards to calorie labels. As far as I can tell they either burn stuff to see how much energy gets released or count up macro nutrients and multiply by x calories. But if you pay attention it should be obvious that you don't digest all foods equally well and I would say the variance can be on the order of 20-30%. Do they account for this when labeling food ?