Why doesn’t the sum of the sugars, dietary fiber, and starch equal the value for total carbohydrate in NDSR?

The values for the three components of the carbohydrate are often taken from a variety of sources, mainly food analyses appearing in the scientific literature. Generally, for a single food entry, the fiber value is from one data source, the sugar values from another, and the starch value from a third. The total carbohydrate value is usually from the USDA Nutrient Data Laboratory database. Since the data were obtained from a variety of sources, and therefore a variety of food samples, the individual carbohydrate fractions do not necessarily equal the total carbohydrate.
Even if the carbohydrate values are all taken from the same source, there may be a difference between the total carbohydrate and the sum of the fractions. This is because the total carbohydrate for 100 grams of food is derived by subtracting the amount of fat, protein, ash, and water from 100 grams. Therefore, the resulting total carbohydrate may include compounds other than sugars, dietary fiber, and starch.
Additionally, the NCC Food and Nutrient Database contains fields for sucrose, glucose, fructose, lactose, maltose, and galactose. There are some other sugars in foods (e.g. oligosaccharides) that are not yet included in the database due to inadequate data.