It's not exactly the individual owners who set the price. The individual owners report their local competitor's price to head office through a automated telephone system. The system calls them back shortly after analyzing the data and tells the individual owner what to set the price at. Prices are usually checked twice a day, everyday. They are very consistent and that's why it seems like they never blink.
They don't want the price to be 1 - 2 cents per litre cheaper because that can trigger a gas war, where the prices spiral and they actually lose profit at the local level.
I was the manager at a gas station when I was younger and am very familiar with the automated system and how the prices are fixed.
I think that you'll see that dynamic across a variety of industrial sectors. Grocery stores are also well known for doing much the same thing (although I can't say if it's via automated telephone system).
My point is that it is not collusion/price fixing per se. For that to be the case, I think that the prices rise but not so wildly over short periods of time.