With a carbohydrate calculation where the use of a sugar alcohol is not being measured, 1g of carbohydrates = 4 calories.  When a sugar alcohol such as erythritol is used, this calculation is 1g of carbohydrate x .2, as the food is a lower energy density and fewer calories per gram.  These calories are only .2 per gram of carb.  Less of the carb is absorbed by the body when a sugar alcohol is used, which results in a different multiple for the calculation.