I think you are right RichC; agencies reserve blocks of cabins for their own promotions. And Princess releases them in a trickle, as you say.
I haven't looked at other on-line agencies, but on the Princess site you have the option of entering a specific cabin number and it will tell you if it is available or not. That's how I've booked my last three cruises.
And sometimes I will just spot-check to get an idea of how full the ship is. There are usually lots of cabins availabe in addition to the ones they put up on the screen.