I noticed that the price of Diesel in my area ranged from $2.27 to $2.99.$22 But stations near one another all had the same (usually lower) price.
I always buy from the cheapest place, because most Diesel fuel is sold to truckers, who pay for it themselves, and buy 50 gallons or more at each fillup.
50 gallons at $2.99.9 IS $149.95, while 50 gallons at $2.27.9 is $113.95. The difference is $36 bucks. So my guess is that the fuel at the most expensive station has been sitting around for quite some time, while the cheaper Diesel is most likely fresh. Water gets condensed in the underground tanks, by the way, so if you fill up while they are filling the tank (which stirs up all the water, rust and crud) you might end up with a clogged filter, or more water in your tank, which means you get fungus of some sort breeding in the layer between the water and the fuel. Yuck!
I think that regulations often get broken, but they tend to be followed when they are in effect.
For example, only VW seems to have been the only auto company cheating on the emission control regs. Prior to that, the ones who used cheater software were big trucks. The cheat was discovered by this one guy at West Virginia University in Morgantown. He knew how to check emissions when the vehicle was on the road because he was monitoring Diesel Coal trucks. I find it amazing that VW thought they would get away with this forever.