The reason is that the truckstop chains have deals in place with trucking companies. Depending on their size and leverage, the trucking companies are paying very little above the truckstop's cost for the fuel. In any case they are not paying anywhere near the posted price. That price is for anyone else who falls into the truck stop and buys diesel.
Gas stations OTOH don't have the same kind of customer base, there is usually some local price competition, so they view the diesel price differently.
This is based on first hand knowledge of the truck stop industry.
