Southwest is offering a $30/$60/$90 deal from Denver and claims the fares are "based on how far you travel." How then is it that SFO is in the $60 bucket while it's $90 to OAK?

It used to be that OAK was the cheap way to fly to the Bay Area, but it seems the fares to SFO are almost always lower these days, at least from DEN. What gives? Should I just be thankful for the Southwest/United/Frontier battle for DEN-SFO?