This is unique to the shorter 3-4 days cruises, I don't know the the rules for longer cruises. Either way, they really shouldn't have these "secret" teirs inside the D+ range. They should simply rack and stack the D+ members sailing by number of nights and then give out the upgrades accordingly. The only cabin they do "exclude" in writing in the program is the Royal Suite, which I can understand.
It's not so much that they have established these teirs inside D+, I just think they need to be transparent to the members on this.