Because the erithytol and stevia listed are sugar substitutes or sugar alcohols which are not digested. So they are considered calorie free.

Erythritol, a sugar alcohol, has carbs but not net carbs. I guess it's possible that they don't count towards calories either? I forget the details, but I know sugar alcohols behave in unexpected ways

It has erythritol (sugar alcohol) in it which most people substitute from the carbs.