I think the recommendation is mainly because there is often more fat in Greek yog (up to about 10% in some delicious brands
).
Yog is made by taking milk (which may be skimmed, semi, or full fat) and adding powdered milk (which may be skimmed, semi or full fat). So there is a lot of variation between brands. Plus the extras like flavourings which can often bump the carbs up more.
And for every gram of increased fat, there is one less gram of carb. ish.
The other advantage of higher fat in yog is that it is slower to digest and more filling, so you get less blood glucose but more satisfaction, which usually means you don't get hungry so quickly.