Single and Return Flight Costs

[IndieSinger]

The last couple of times I've flown, I've used Expedia.co.uk and both times, booking a return journey was cheaper than booking the single (by roughly 100% each time). Why is this? Is it literally because a return ticket is a guaranteed 'bum on seat'? I'm in Germany now and when I came, I booked a return (but didn't use the return ticket) and when I go home, it will be 50% cheaper to book a return again (and not use the return ticket again). The return (and direct) flight is around �160 and the single is around �330. Surely people don't both buying ANY singles if they're twice the price of the return? Is it just expedia?

[sft42]

Partly you are wight with the bum on seats theory and and partly for the reason that if a return ticket isn't used then once the check-in for the flight has closed anyone waiting on stand by for a seat on the plane can the buy the vacant seat.....the airline then gets two lots of cash for the one seat on the plane.