DL does alot of playing around with which classes are even available. I've been monitoring some flights to Alaska in July/Aug. Some times, there are only fares in K class or higher and at other times, L or U or T fares show up on that routing, only to disappear after another day or two. In fact, on some of these connections, Boston-MSP alone is an L fare but Bos-MSP-ANC is a Q or K or H fare, same flight BOS-MSP.

Seems like DL is using a complex algorithm for maximizing their revenue and this includes psychology such that when they drop a high fare a little, people will buy, even if waiting a few more weeks might lead to a much lower fare. Fares are higher when bought on weekends for some routes because that's when people have time to look for flights, and many don't know how fares go down as well as up when you get closer to the flight day.