The margins the FDA allows for class 2 and third group nutrients are also quite generous. I'm sure they made sense back when they were first introduced, but as food science has improved, the standards have not.

> The Food and Drug Administration (FDA) allows calorie content to exceed label calories by up to 20%

https://pmc.ncbi.nlm.nih.gov/articles/PMC3605747/

> Class I nutrients are those added in fortified or fabricated foods. These nutrients are vitamins, minerals, protein, dietary fiber, or potassium. Class I nutrients must be present at 100% or more of the value declared on the label

> Class II nutrients are vitamins, minerals, protein, total carbohydrate, dietary fiber, other carbohydrate, polyunsaturated and monounsaturated fat, or potassium that occur naturally in a food product. Class II nutrients must be present at 80% or more of the value declared on the label.

> The Third Group nutrients include calories, sugars, total fat, saturated fat, cholesterol, and sodium. [...] For foods with label declarations of Third Group nutrients, the ratio between the amount obtained by laboratory analysis and the amount declared on the product label in the Nutrition Facts panel must be 120% or less, i.e., the label is considered to be out of compliance if the nutrient content of a composite of the product is greater than 20% above the value declared on the label.

https://www.fda.gov/regulatory-information/search-fda-guidan...

Edit: Expanded the quotes to include definitions.