I think you'll find it's just a peculiarity of their res system. The flights get loaded at the highest fare and then opened downwards as their revman guys work through the flights. Happens every year. Once the flights are all done they are officially "released" through an announcement on the website and by email to their mailing list. The only bases that get the chance to be set-up in advance of release are the first ones on sale.
For anyone who bothers to read the website...
Summer 09 routes now available from Luton and Birmingham
Plan ahead - Summer 09 routes are now available from Luton and Birmingham, book your summer holiday early and save!