Well, they have been extremely low for a long while now. It is purely based on supply and demand for any particular cruise on Carnival. They actually adjust prices up to 4 times a day depending on the cruise.
Sometimes Travel Agents will have the price advantage by having group space reserved at a lower price. So I always try to go with a reputable Travel Agent that specializes in cruises as they have access to the same prices that Carnival has listed directly. This is just my opinion from past experience.