Airline pricing is very complicated. It's about so much more than 'filling up' an airplane. That aside, there is an old trick some airlines use on their website when you book tickets. I'll try to explain.
You want to go from point A to point B. You like US Airways so you go directly to their homepage and search for your route and it gives you a price. Then, obviously, you search the net and other airlines for a cheaper ticket. You come back to US Airways, they remembered your IP adress somehow and the website thinks 'Hey that person REALLY wants to go there, let's increase prices a bit". You see the new price, get confused, look at the other prices again and when you come back again, they raised prices again.
I'm not saying all airlines do it but it's a trick especially used on medium to short flights at which the company knows they won't be making a lot of profits on.
A second thing you have to keep into consideration is that the earlier you book, the cheaper the flight. If you book one or two days in advance, the website will think the same as above. You'll pay more because you HAVE to go on that day.
Business travellers and First class passengers have enough money to spend that they don't care. (In general)
All airlines rip you off. No exceptions, cause flying is sexy and cool so people pay for it. If you don't want to be ripped off, don't go.
I'm not trying to make it right but it happens, so you might as well accept it because it's not going to change.