Okay, I'll take a stab, but I am only guessing here.
The difference between those milks is the amount of fat. Fat and Fiber slow down the speed with which your body absorbs simple carbs, like the natural sugars in the milk. So the extra fat in the milk must make you absorb the lactose more slowly, making it lower GI. However, the fat is a saturated fat, and we are to stay away from those and eat unsaturated fats. I think in this case, the higher fat milk may be lower GI, but the lower fat milk is probably low enough on the scale, and the absence of those fats makes it more healthy overall.
Life's a journey, not a destination.
It's easier to stay on plan than to get back on plan.