Different kinds of Atlantis coils?

The recommended wattages for the Atlantis kanthal coils are (1.0 Ohms): 20W - 30W, (0.5 Ohms): 40W - 50W, (0.3 Ohms): 70W - 80W

But the coils that I use are .5 ohm and rated at 20-30W? What's the difference between these and the ones rated at 40-50W? Maybe one is a single coil and one is a dual?
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