The difference is that for command, very few smoke units are wired directly to track power, so the fact that they have 27 ohm elements doesn't cause excess power dissipation as they're not running on the full voltage.
Try this simple experiment. Wire one of those 27 ohm resistors to your transformer on the bench and set it to 18 volts. Before long, you get a very hot resistor, far more than is necessary for a smoke unit. Even if you don't reach the flash point of the smoke fluid, you'll char the wick very quickly, it's simply too much heat.
