Commercial yogurt makers stop fermentation at a certain pH because they don't want it too sour. Then they add pectin or cornstarch to thicken it.

They measure the carbs at the end of the manufacturing process, not when the stuff hits the store shelves.

I'm not an expert on yogurt culture. I just wanted to give some leads to those who want to pursue this. But if you want strict carb control, you're better off making your own.