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.