I expect it’s that skim has the same amount of lactose sugar. But, with less fat you’ll likely drink more of it before being satisfied.

Everyone should try lactose-free milk just once just to see how very sweet it is. It has the same sugar content as regular milk, but regular milk’s lactose is a form of sugar that we don’t taste as strongly.

Lactose free milk does tend to have a bit less sugar overall than regular milk - something like 3.5% sugars instead of 5%.

I don't personally notice lactose-free milk as being any sweeter than regular, and it doesn't look like galactose is any sweeter than lactose?