The water cans are the same size and shape, but only have one handle. So you can tell whether you're picking up a fuel or a water can in the dark. Useful :)
“original” jerrycans don’t have that distinction, because the purpose of the extra handles are even more so for water: carrying two empties in one hand doesn't change, but the ability to share one can between two people is even more useful when carrying 20L water (20kg) than 20L gasoline (15kg).