Why does low-fat and fat-free milk have more lactose/milk sugar?

The difference in lactose (milk sugar) content in fat-fee milk and full cream milk is marginal (less than 0,17%).

Fat-free and low-fat milk has less fat than full-cream milk, i.e. 1,5% in low-fat milk and 3,3% in full-cream milk. Therefore the ‘water’ portion of milk (including all the water-soluble nutrients), in the fat-free milk is slightly more to make up the difference in volume of the water-soluble part of the milk (1,5% to 3,3% fat). Naturally, the lactose content will be marginally more in milk products with a lower fat content than in full-cream milk products.