I don't know why....it's really pretty simple. Their system is called "yield management" and its goal is to not leave any $$$ on the table when the plane takes off. Their computers know exactly how many seats on each flight, to each destination, on every day of the week, every month of the year, should be sold 21 days out, 14 days out, 7 days out, 3 days out, (actually hour-to-hour) right up to the time of departure.
If, for example, they know that 21 days out Flight 1234 to Seattle should already have 50 seats sold, but instead has only 40 seats sold, they'll offer 10 discounted seats to get ticket sales back to par. Then 14 days out they'll look at the numbers again and discount a few more to get them back to par if ticket sales are lagging, or if they're ahead of projected ticket sales they'll offer no discounts at all.
Every day, every hour actually, the algorithm will tell them whether to hold firm on ticket prices or sell 'em cheap. In theory if just a few days out they see they're way behind filling seats they'll panic and offer some great last-minute deals.
That's one way the travel sites like Expedia and Travelocity can offer the deals they do. The danger is if you wait that long and they're not offering any discounted seats (and you have no way of knowing) you'll have to pay an exorbitant last-minute price. Ouch!
There really isn't any way to outwit their computer. Your best bet is simply to check prices often and fly at the most unpopular times of the day on the most unpopular travel days of the week.
Or date a flight attendant and have her fix you up with a "buddy pass". *wink*