This simply doesn't make economic sense.

[randomness]

It is kind of odd when two one way flights are cheaper than a round trip to and from the same airports on the same days.

Not unheard of, but odd.

Comments

[drwex]

My guess is that it's a search optimization. By removing one-way flights the search can be done much faster. As you noted, it's rare that two one-ways is cheaper so it's generally a reasonable optimization, I think.

[nathanjw]

Not even close, as it turns out. Searching for a cheap one-way from A->B and a cheap one-way from B->A is vastly faster than searching for a cheap round trip from A->B->A; enough so that if you're doing the latter the former is in the noise. In the former case you do two (relatively) simple searches, but the combinatorial complexity of the latter (particularly as regards the airline fare rules) is much, much worse.

(I work on Google Flight Search, with a bunch of the people and software that Google absorbed from ITA Software. Our job would be much, much easier if composing one-way flights made sense most of the time.)