Your post sent me to look at the Total/Fage Greek yoghurt (full fat version) I eat, and the carbs/sugar content in this is 3.8 gms per 100gms, and the 0% fat version is only 0.2gms more at 4.00gms per 100gms. That's a huge difference to the 19gms of carbs quoted! Both these versions contain the same live cultures (L. Bulgaricus, S. Thermophilus, L. Acidophilus, Bifidus, L. Casei), which stay live until the expiry date. The low fat is made with skim milk, and the full fat with pasteurised milk & cream.
Confusing isn't it!!
It's well over 30 years since I made own and as far as I can remember I used to start if off with a dollop from a bought pot.
Robbity