We don't get it from raw bomb calorimetry ("Burn it and measure how hot it gets"). There are additional absorption coefficients applied before the nutrition label is printed. But for proteins, fats, and simple carbohydrates the coefficients are quite high (~95%), while progressively more complex carbs have a declining curve that ends in indigestible fiber. The difficulty is that the same person will digest more or less of the protein in their hamburger depending on how many fries they ate, how greasy those fries were, whether they ate lunch, and how much beer they drank that night. It's variable.