From the small amount of difference, it's quite likely the travel agents have booked group space, which normally does offer a small discount of the "going rate" by the cruise lines.
If travel agents are offering onboard credits, or other amenities, they are rarely paying for it themselves. Rather, there are "points" assigned to different sailing where agents are allowed to use those point to choose from a menu of amenities, which might include the onboard credits.
It's also possible the travel agent booked group space, and later the price rose, so they are offering the lower rate.
With group rates, after a period of time, and especially if a sailing is selling really well, the cruise lines will take back cabins in a group that haven't been sold.
Some agents with group space will actually sign a contract, and actually make the deposits on all their group cabins. In those cases they can keep the lower priced cabins for a long period of time.