The 20% seems to be how much you are allowed to lie by. You get another bit for variability of the test, and a third error term for variability of "good manufacturing practice".
A food with a label declaration of calories, total sugars, added sugars (when the only source of sugars in the food is added sugars), total fat, saturated fat, trans fat, cholesterol, or sodium shall be deemed to be misbranded under section 403(a) of the act if the nutrient content of the composite is greater than 20 percent in excess of the value for that nutrient declared on the label. Provided, That no regulatory action will be based on a determination of a nutrient value that falls above this level by a factor less than the variability generally recognized for the analytical method used in that food at the level involved.
Reasonable excesses of vitamins, minerals, protein, total carbohydrate, dietary fiber, soluble fiber, insoluble fiber, sugar alcohols, polyunsaturated or monounsaturated fat over labeled amounts are acceptable within current good manufacturing practice.
I am not a lower but the 20% do not seem to be about carbs. Only sugar. Carbs are only covered by the second paragraph and must be 'within current good manufacturing practice', whatever that means. Am I reading this wrong?
The toothless FDA allows for a 20% margin of error on nutrition facts labeling, so it could possibly be one or the other or both.
Some products may just have variation. Some foods will be maliciously mislabeled with 19% less calories/sugar/fat but may have little to no variation within the same product.
Is this accounted for by product-to-product variation or package-to-package variation?