While I agree that in general, Carnival appears to be less expensive than RCL, it really isn't fair to compare your two cruises price-wise unless they were very similar in length, itinerary, dates traveled, and type and age of ship.
It would be like saying, "Marriott is cheaper than Hilton because I paid less for my 7 days in an old Marriott in Detroit in January than I did for my 5 days in the brand new Hilton near Disney World in June".