It could have been a very simple mathematics in the end for buying cheap air tickets for round trips. Add the prices of the flight tickets for the each leg of journey and you get the price of the tickets which would be your price of air travel on that trip. Unfortunately, this is not the way the prices are calculated by most of the airlines since their revenue and yield management methods do not allow such simple mathematics to have a play in the pricing of the air tickets. A number of ifs and buts and historical trends make the price calculation quite complex.
What makes the round trips generally cheaper? Two of the factors on which the airlines price their flight tickets are the route segments and the profile of the passengers. The route segments which are see high-demand and high-competition are likely to see a reduced round trip tickets so that more people wean away from other airlines. In the high demand and low competition sectors, the airlines have a general preference to price the tickets in the same way as the single-trip ones but in order to retain customers, there might be some reduction in the prices of the round trip as compared to the added value of single trips. In conditions where there is low-demand, the prices of the round trips are valued at very low level since the airlines are primarily concerned with filling the planes with passengers. So, the best prices of the tickets can be obtained on these sectors.
Airlines also understand that some routes are essentially taken by the business travelers and in order to ensure that the planes are filled by the high yielding business travelers the prices of round trip cheap air tickets of these segments are priced at almost the same level as that of the single trips. So, the primary concern of the airline is to fill the planes and, if that can be done by complex pricing of the round trip flight tickets, in a way different and cheaper than the single trip ones, than it results in better revenues.
When you need to cancel the tickets of any leg of the travel due to some reason, you shall read through the policies of the airlines governing the same before booking the flights on that sector.