Perhaps all the flour, yeast and hot conditions muddle bakers’ brains so they can’t count properly? Or it could be a remnant of a law in medieval England. The Assize of Bread and Ale, passed in the reign of Henry III, regulated the price, weight and quality of bread (plus beer). Any baker found overcharging customers was subject to harsh punishments, including fines, jail and beatings.
A 13th-century baker had to use their loaf. At a time before scales, they had to ensure they gave enough bread for the cost, so chose to add a bit extra, such as a whole loaf, to their dozen – preferring to take the loss than be flogged.