I assume that you are talking about either Regatta, Insignia or Nautica and I also assume they mean the sofa which, similar to the cabin, is not huge and unfolds to make a small double bed so the cabin would then hold four people. I do not believe that there is any likelihood of a cabin holding 6 people unless it's a suite. .... Neil.
Not sure if Oceana has this facility but Fantasy does have quad rooms with a double sofa bed as an option. If you go on one of the newer boats, you can also get quad rooms with a single sofa bed and one upper. In other words it sleeps 6 but a long wait for the shower !!!