Why is it cheaper to cross the channel on a return ferry crosssing when you spend a shorter time abroad?

Answer 1

I think that it is because you don't have an open ended ticket, so it is easier to judge how full the ship will be.

What is strange though, is that 2 adults and 2 toddlers go cheaper than 2 adults.
Just tried on-;line quotes, The wife and I plus 2 toddlers is £45.00.
The in-laws (2 adults) are £65.00