Not That I'm Complaining, But

Can anyone explain to me why gas is $.10 a gallon cheaper in Williamsburg than it is in Iowa City? It's the same question that's always bothered me about Cedar Rapids, except now I can get the cheaper gas without driving someplace I wouldn't otherwise be driving. You would think that there wouldn't be any additional cost factor to delivering gas to Iowa City that doesn't also apply to delivering gas twenty miles away in Cedar Rapids or Williamsburg. So what is the difference? Anyone? Anyone? Bueller?