Comercial yogurt contains between 11g and 18g of carbs (all sugar, all "effective") per cup (according to the USDA database).
If you buy "natural" yogurt it is made from milk and culture, and it is typically very "watery" and breaks up into liquid when you stir it. So a cup of milk (14g carbs) turns into yogurt (11g carbs)...
The way maufucturers make it more creamy is eiter draining it (remove water), or start with concentrated milk which is made of milk and dry milk powder. Either way, you will get a lot more carbs per cup...

So, if the yogurt you are making is lush and creamy it has lots more carbs then "natural" yogurt which is high to begin with..