We booked through Travelocity for our 9/18/04 Cruise. At some point the block of cabins that this site showed "sold out", but the Princess site showed what cabins were available. So, the hunch is, that each of these travel sites have so many cabins they will, or can, fill.
Our other hunch is that it's better to reserve a cabin through Princess itself so that there is a potential for an upgrade.This is our 4th cruise with this line (out of 5) and we got our inside cabin in February because of a great deal through Travelocity. However, with the cabin already assigned with a number, it's doubtful that any upgrade will materialize. Lynne (We' re happy just to be on the ship!)