I'm not a expert, but when a cow is milked it goes through a process of how they sell it, pasteuried, green top, red top and so on, even to condensed milk. (Yuk)
The milk fed to calves is full fat, the skimmed milk has most of the fat took out or watered down as you say, and if you drank that it would taste like very bland.
So to pass regulations and achieve taste, the fat which gives it taste, is replaced by additives.
If you ate a yoghurt which is no fat, low fat, during the process, depending on which brand you buy, without the fat it is awful, so they add fruit to taste or an enhancer to taste nice.
So milk straight from the cow, is full fat, and very few carbs, The more they add after taking the fat away, the higher the carbs.
So if you take milk in your drink, add full fat or cream if it's coffee. Or like me, none at all because I'm lactose intolerant!