Why can't I book a four berth cabin?

I was speaking to a customer yesterday who was trying to book a last minute deal with Royal Caribbean cruises for a four berth family cabin. The cabin they wanted was available and would sleep four people. However Royal Caribbean would only let them book two people into the cabin. They wouldn't sell the cabin my customer wanted as a four berth.
I explained to the gentleman why he couldn't book it but he wasn't very happy with the explanation.
It occurred to me though that a lot of other people might be confused about this issue so I thought I'd explain it in todays post.