Ok, maybe I dreamt this but I swear I read this somewhere...
The "mattresses" are double the twin size but when making up a "twin" bed, they are folded in half and placed on the frame - think of a taco shell. When making up the "queen" bed the two mattresses that come from the two "twin" beds are not folded and are stacked one on top of the other - think of a tortilla. This means that either size bed has a double thickness of that special mattress; double on the "twin" because of the foldover and double on the "queen" because of stacking the two mattresses.
Clear as mud???
Hopefully someone can confirm that so I don't wonder where my mind got it from.
Of course, this may just be on newer ships or another line.