That's true, but it terms of the size of tank needed, the guidelines are helpful. Putting a fish in a tank where it's unable to turn around without smacking into the side is a bad choice, for example. I can setup a 20 with more than enough turnover to keep the water oxygenated to support a huge fish, but there's still not enough room for the animal. There are many components to consider when stocking. The size of the animal, the dietary needs, the bioload of the tank, the lifespan, the tank mates, etc.