Costco will give you a sampling of what's available, but they are TAs, not resellers, so any room that's available on DCL will be available to book with Costco. You might have to call them to get your desired room and they will call DCL to make the booking.
Curiously, I've found (unless it's been recently fixed) that deck 9 4As on Costco always look unavailable, even on opening day and when they show up on DCL. It's definitely a glitch in their programming, as the rooms are available for booking, just not through the online system.