I really do think they open blocks of rooms at a time for different specials. There's also an issue of if their system is being glitchy.
I've had times where rooms were sold out then all of the sudden some would open up. I've had a CM look for rooms and they couldn't find any but then a few days later some would open up.
If you really want to go, I'd say book a room only where you can and where you can afford it, then change if something opens or change dates if you need to. If you book room only, the cancellation is more lenient than a package.