CAT - Are my cats underfed, or do Purina's numbers not add up?

Not really a cute cat moment, but that seemed the best category anyway.
I have five cats. One has been thin since we got him, one is maybe a little overweight, though I tend to think he's a healthy weight, and the other three are chubby to fat. Every day, they get a large bowl of dry food, a can of human tuna, and two cans of cat food (or a can and a pouch). None of them are very energetic, but they aren't very lethargic either--they seem perfectly normal to me. Also, no one has ever said "is that all you feed them?" or anything to make me think there was anything wrong with their diet. The vet has said nothing. The cats eat almost all their food, but not quite all--which seems to back up my impression that they get the right amount of food.
So, imagine my surprise when, while opening the can of catfood tonight, I happen to read the fine print, and it says, "Daily ration for an average adult cat is 1 can per 3 pounds of body weight." If that's true, one can per three pounds of body weight means the cats get only a third to a quarter of the food they need. That begs the question--if they are so badly underfed, why don't they even finish what they do get? Further, I would think that none of them would have very much energy, and all of them would be thin.
I'm mystified--the numbers don't seem right to me. The dry food offers more volume--so maybe it's not as little food as it seems to be. I can't imagine the cats would eat that much even if I offered it to them. So why does the can suggest so much?