Why are one-way tickets....
 

....more expensive then return tickets sometimes.?

The answer to your question (Legacy carriers only) is a VERY long one. It dates back into antiquity (ie 1950s!) and the very complicated rules by which govts managed their nationalised airlines.

These fares remain - even though many folks book the return and do not make the return sector - for a multitude of reasons and will eventually fall away as we move into the direct booking and on-line world.

BMI, SAS and BAConnect, all claim to offer one way fares, but I've never compared costs.

BA now offer one way fares at sensible prices. A year or so back the cheapest one way fare I could find between Gatwick and Manch was £170ish via Edinburgh. Just booked the Gatwick to Manch directfor £28.50 all in for travel in 3 weeks time.

Mind you, a one-way ticket is usually a warning flag for security.