What Doppy said. The airlines are very good at determining how many seats they can sell and how much they can get for them. Which is why they have such a plethora of fare bases. For people who need refundability or flexability, they usually offer a fare 3-4x what the cheapest is. But that fare is only good until about 1-3 days before the departure date, and have a fare 2-3x that fare which, while available from 330 days out, is usually only sold 1-3 days before the flight departs when it is the only fare available.

Below that are fares that run the whole gammut, usually broken up by requirements such as must stay over Saturday night, or are only good for departure or return on a certain day (or days) of the week, or require a longer or shorter stay, etc.

This way, they can offer the "lowest" fare to the leisure traveller going on vacation for a week. Then charge more for the traveller going on vacation for a weekend. Then for the business traveller who needs to depart on Thursday, but can come home Monday. Then for the business traveller who needs to depart on Thursday, but must come home Sunday.

And so on, and so on, and so on.

It's actually a pretty effective system, except that the LCCs can offer their maximum fare at a price much closer to the minimum the legacy carriers can, which is where they lose their shirts. On routes without LCC competition, they make a bundle (until such time as the LCC comes in, because there is so much pent-up demand for lower fares).