Delta rates/dates/schedules are out because like the other legacy airlines, they release fares 331 days in advance. However, while the fares may fluctuate between the release date and the flight date, the schedules almost certainly will change. Just see the complaint threads about Delta, Northwest, etc, changing/cancelling passengers' flights.
Discount airlines like Southwest, JetBlue, Airtran, release dates and fares on a much more limited basis BUT they're pretty certain of the actual schedules (which, I think some of the experts here indicate, are generally finalized three or four months in advance).
Also keep in mind, fuel prices soared over the last six months or so; even with fuel contracts, I wouldn't be surprised if the discount airlines were being a little more cautious before determining fares.