Airlines have many different ways of setting prices on their tickets. It depends on the route, seat availability, how long you are staying etc. These rules change from time to time, and a lot of it is down to blind luck.
The trick is finding out what set of rules result in the cheapest tickets. It used to be the case that booking a return ticket was cheaper than a one-way, so people could just not show up to the return flight if they wanted a one-way or multi-city trip. It also used to be the case that waiting until the last minute resulted in cheaper flights, once you weren't too bothered about where you actually went. This is still true in certain cases if a travel agent or tour operator has bought airline seats in bulk and needs to get rid of their remaining stock.
A few years back, there was a lapse in the federal taxing system in the USA which resulted in the tax on airlines expiring and resulted in hundreds of passengers getting very cheap transatlantic flights, and there have been numerous instances of computer errors that resulted in cheaper flights.
But there are ways and means of getting consistently cheaper flights, taking advantage of the airlines own pricing rules.
I have come across this knowledge, to a certain degree, from working as a travel consultant, but for those who have not worked in the travel industry, there is a much better way to learn. There is a book available, written by Henry Rustkirk that will give you all this information, and more. I would recommend anyone to get it and read it, it could save you a massive amount on your next holiday! Check out the Guide Here.