If you're suspecting fraud from the station, I saw something about that on TV a few years ago. There was a place out in CA (if memory serves) that had some complaints against it, but every time the Dept of Consumer Protection went out to check by filling their 5 and 10 gallon official tanks, they couldn't find anything. In the end someone found a computer chip on the inside of the pump that was programmed to make the 5 and 10 gallon amounts accurate to fool the inspectors, but fraudulantly bumped up the gallon count at other quantities. They said it took a computer scientist to figure out what the thing did once they found it. They station owners were charged for this.
I can't imagine there are too many guys out there pulling this stunt, though. I may be naive, but I'd have to guess that the majority of stations are on the up and up.