Gas prices have a lot to do with what type of place you go to. Corporate owned chains can open a new store and sell gas for a loss, and just make up for it in profits from other stores. In recent years large grocery stores have been selling cheap gas for a loss as well. The theory is people will come for gas and stay for groceries. In Wisconsin its actually illegal to sell gas for a loss. but that's not the case in Illinois. Gas stations in IL can set the prices to whatever they want. That can make it difficult for independent owners to compete with the larger operations selling gas for a loss.
There is a move to change the Illinois laws to help independent business owners. If that happens we could see more uniform gas prices. Until then the gas wars, and widely varying prices will continue.