It appears that there has been some mixup in communication between AMD and its partners, which resulted in a bit more ROPs on the HD 6790 card. AMD's official specs stated that this Barts GPU should be left with 800 Stream processors while the number of ROPs should be 16, or half of the fully enabled Barts GPU. Unfortunately, or luckily, some if not all partners shipped their own HD 6790 card with 24 ROPs which gives a bit more geometry punch to the HD 6790.
It isn't the first time that AMD or its partners "unintentionally" mixed up the specs of some card so this really doesn't come as a surprise. AMD and its partners might change this on its next batch so don't get you hopes up unless you already snatched a HD 6790 that has 24 ROPs. We still don't know how these cards ended up with extra ROPs but we guess that more info should surface pretty soon and AMD might have a thing or two to say about this situation.
Credits go to Techpowerup for this one as their GPU-Z was the first clue to find out the 24 ROP info. You can check out more here.