I long ago gave up trying to make sense of travel costs. A flight from Heathrow to Orlando is £120 dearer than a flight from Edinburgh to Orlando, which flies down the Heathrow and connects with the same flights!
Doesn't answer your question but shows that anomalies occur all over the travel sector.
I know that "Full Board" on cruises can end up costly as you get a hefty drinks bill at the end!!
You will still spend on a cruise ship as all activities come at an additional price. The cruise companies know that and their All Inc prices are often enticing.