It probably has to do with the agencies contracting with the cruise line for a "block" of rooms, in order to offer a group rate. If different websites are offering the same exact cabin numbers, then either they are all part of one company, or else these are the only cabins that are NOT part of other agencies' group allotments. Most of the cruise lines allow an agency to hold a block of cabins for only a specific amount of time (which varies with the cruise line). If those are not sold by the agency before that date, the cruise line "recalls" the unsold cabins and will try to fill them through other means - either by themselves or through a different agency.
You might try working with an in-person travel agent in your area to see if you have more cabin choice. If not, I'd suspect my latter guess was true.