Family of 5 - why can I make reservations at Dolphin but not Swan?

[mom3girls2007]

As a family of 5, (kids 10,10 and 3) I have been looking for a great place to stay at Disney in May 08. I decided on the Dolphin or Swan because of walking to Epcot and proximity to Boardwalk, MGM, etc. without high cost of BC or YC. However when I go to the reservation system to see what they will cost, I can't book Swan because "too many people per room" but can book Dolphin. It doesn't make sense, since Swan has queen beds and Dolphin doubles! Does anyone know why? Are we going to regret staying here and not BC (but saving $600 over 6 nights on the room?) Our other choice is POR, but Swan/Dolphin looks much nicer. Last (and only other) time at Disney 5 years ago we stayed in a savannah room at Animal Kingdom Lodge -- LOVED! it but now our party is too large and suites are too expensive.