Yes, those group cruises are more expensive than the price that you would normally pay for the same cruise. Atlantis Events ends up chartering the entire cruise ship for that sailing and brings onboard their own entertainment, dj's, etc. This cost ends up being passed on to the end consumer.
Of course, it's also a law of supply and demand. Those cruises always sail full and sell out early, so they know they can get the premium that they charge.