I have made enough online travel queries to know that there are severe quirks in the system.  Two of the more prevalent lead to illegal, at least according to the airlines, practices called hidden city ticketing and throwaway ticketing.

Hidden city ticketing is often pursued by value-conscious travelers because flying from Buenos Aires to Toronto with a stop in New York may be cheaper than the first leg of the route alone from Buenos Aires to New York.  A person wishing to fly to New York may purchase the fare to Toronto without any intention of completing that leg.

Throwaway ticketing results when the round-trip fare is cheaper than the corresponding one-way fare.  I often check the round-trip version of a one-way flight I am interested in to see if there are savings to be had.

The travel search engines could make extracting these savings easier, but these practices are actually illegal.  I understand that the supposedly efficient market forces determine these prices, but there is a lot of room for more accurate pricing if round-trip fares are occasionally priced less than their one-way counterparts.