These signs also exist in the Netherlands, and the reason is because pumps are calibrated to a certain accuracy (say, 1%, for convenience).
But in the real world, you can get a lot of variation from temperature, to how long it's been since a pump was used, to how full the underground tank is. They all made a difference, mostly at the start and end of pumping.
So you get a law in percentages, and you get a real world deviation in volume. Obviously, if your pump is short 50ml on 1 liter, you're off by 5% and breaking the law. But of you slap a sticker on, telling everyone they "must" get 5 liters, you're off 50ml on 5L, a perfect 1% deviation and entirely within the limit.
In Canada ours have a sticker that says the amounts are calibrated to a certain temperature (15C I think), so I assumed it took that into account. I try not to think about how much it costs so didn't over analyze it.
Or they were just taking it home to store. Which, y'know, will likely result in that gas eating the plastic bucket in a couple weeks because gasoline is incredibly good as a solvent.