If carbs and protein are 4 calories per gram, and fat 9 calories per
gram, why do the total calories sometimes = less and sometimes more
than these three added up? They rarely come out equally and in some
cases quite discrepant. (look at
several food items and you'll see). How are they determined?

1) Some degree of rounding is "allowed", legally.
2) Sugar Alcohol (7kcal/g, not required to be on nutrition labels)