yesterday I filled up my Civic out of town, at an Exxon station, after I pulled out, I passed a Mobil station less than half a block away. Their product was priced 2 cents higher. Do you think it is the same product?
I don't think ExxonMobil provides different blends to each station as it would probably mean increased distribution costs. Also, if you check out the gasoline FAQ's at the Exxon and Mobil websites, they are word for word the same.
Independent distributors can charge whatever they wish. You could have five Mobil stations in a row and all five be charging different prices. Up to the owner/operator.