I imagine that the sugar is lactose but from what I know it is not measured directly in yogurt but just is the amount contained in the milk before fermentation, so the sugar content may be less than what is listed on the label since the process of fermentation eats up some of the sugar.

I used to make my own greek yogurt. I fermented half and half for 24 hrs in a yogurt maker and then strained out the whey. I used half and half from a local dairy. It came in glass bottles with no added anything like most of the commercial products have. I had no precise idea how many carbs were in the yogurt and could never find a good answer to the question. I just knew that the longer the fermentation time the less sugar so I estimated it at half the original sugar content.

So there's my long winded explanation. More succinctly put: Who knows?

Jean