Why on the Bread content labelling total % may not add to 100

In case of bread, as in many other cases, the contents %s may not add up to 100. One may see contents being mentioned in decreasing order of ingredients. The difference is due to insignificant -ve health effect of the remaining ingredients.  In case of  “Bread,” USDA conducts research on all “ingredients and contaminations” in minute detail to determine human edibility, where as Canadians provide labelling guidelines.