Apparently, the Deluxe Balcony staterooms on the Royal class ship are Guarantee only for two passengers. I think they keep them open for 3 or 4 passenger bookings, and then allocate them to the bookings with 2 passengers as the sailing date gets closer.
We booked a 10-day sailing on Enchanted for last February in October last year. The only balconies available by then were a Deluxe Guarantee (we had wanted a regular balcony) so we went with it. Even though it was DF category we were assigned a stateroom with the larger balcony on Dolphin (D4) about a week before.
So if you want a Deluxe, you have to be willing to accept a Guarantee. I've read where some people have found available Deluxe cabins by doing a mock booking for 3 people, then contacting a PVP to book...