Location Location Location. That is the only difference. Cat 12 is located on Deck 2 where Cat 11's are on decks 5, 6 and 7, they are exactlly the same room just the Cat 12's being on the lower deck are less expensive. (It is just like the Cat 8 and 9 are the same and Cat 5 and 6 are the same...price difference is just for location).
I'm not looking at a deck plan right now, but it seemed that the Cat. 12 cabins were pretty far forward and/or aft. I don't think they have anything mid-ship.
I was told by a DCL rep that the Cat. 12's only sleep 3, some Cat. 11's sleep 4. She booked our family of 4 at the Cat. 12 rate and said we would assigned at least a cat 11!
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!