The numbers the US uses for calories in fat, protein, carbohydrates and alcohol come originally from a man named Wilbur Atwater who around the turn of the 20th

The numbers the US uses for calories in fat, protein, carbohydrates and alcohol come originally from a man named Wilbur Atwater who around the turn of the 20th century calculated them by measuring how many calories someone was consuming and measuring the calories in their feces. They've been modified over the years for specific ingredients, but we still use the Atwater system today because it is very easy for food manufacturers to use. Europe and elsewhere use Rubner factors which are slightly different.