Labels are not foolproof. I have often refigured the numbers on labels and found hidden carbs. There are only three sources of calories in food - fat, protein, and carbs.

The formula is: Number of total calories per serving - fat calories-protein calories-carb calories = zero.

To get fat calories, multiply number of grams of fat X 9 calories
For protein, multiply number of grams X 4 calories
Carbs, multiply grams X 4 calories.

For example a food with zero carbs lists per serving:
80 calories per serving
Fat calories 45 (5 grams x 9cal./gram)
Protein cals. 28 (7gr. x 4 cal./gram)

Leaves seven calories that are unaccounted for. I assume that those are carb calories.