I think it has a lot to do with supply and demand and availabilty of the cabin type. The Splendour came out in 1996 and the balcony craze didn't really start until the 2000's. The NCL Jade came out in 2006. The majority of ships built in the 90's had limited balconies and suites in comparison to the ships built in the 2000's. When the Carnival Spirit came out in 2000 it was one of the first ships to have 80% of cabins as outside cabins and 80% of the outside cabins were balconies. Therefore; the Splendor's limited supply of balconies get a higher per diem. The NCL Jade having much more balconies and suites to fill, has to have an attractive price to fill them all.
Good point, Cruise Fanatic. Anyway - we have loved Our cruises With RCCL, and look forward to spending a week on the Liberty this summer, and for sure it will be great to "rejoin" NCL for a week on one of their fine ships :)