Here is the nutritional breakdown from a milk producer website. As you can see they list carbs as 8 g per cup. Nothing is added to the milk except vitamins and Lactase. Lactase is the enzym that allows you to digest milk, so I assume it's added to help people with lactose intolerance digest the lactose that's left.

So where does the 8g carb come from ? Obviously not from lactose

Natrel Lactose Free 2% milk

Nutrition Information
per 250 ml serving
(1 cup)

Energy 122 cal
Protein 11 g
Carbohydrates 8 g
Fat 5.1 g
Cholesterol 20.5 mg
Vitamin A 110 RE
Vitamin D 88 UI
Riboflavin 0.4 mg
Vitamin B12 0.9 mcg
Calcium 319 mg
Phosphorous 242 mg
Potassium 154 mg
Sodium 50 mg

Ingredients:
partly skimmed milk, lactase, vitamin A, palmitate, vitamin D3