Unfortunately, I can't get a source to it, but from what I remember, the margins on gas are already so thin that most profit at gas stations come from the random other stuff they sell. I mean think about it... 20 cents (maybe) for a gallon of gas, or like 80 cents (or more) for a can of pop.
Also, a side effect of the razor thin margins, as well as the relatively fixed consumption pattern means that while price at the pump tracks crude prices going up almost instantly, on the way down, every single gas station wants to be able to enjoy their extra margins for as long as possible, so they reduce their prices slower. And this clustering phenomenon just encourages that even more, since the moment you undercut your competitor, they'll know in like 5 minutes or something and match you, and now both of you just missed out a bunch of extra cash.
Edit: silly me, linky here: [1] http://energyalmanac.ca.gov/gasoline/margins/index.php
I also remember a radio station's competition where the petrol station that sold fuel at the lowest price won the competition. In the final hours of the competition, there were two or three stations selling petrol at 1c/litre... and they lost to the station that was paying customers 1c/litre...