270 degrees is used as a convenience in differentiating between 270 wheels and 360s. In reality, if you grab 10 different games with "270 degree" wheels and measured their range of motion, you might get 10 different measurements, and may be none of them that are exactly 270.
As for the gears, lots and lots of wheels(incl. Star Wars) have them. (All? I've never SEEN one without.. ) They aren't there to make the pot move exactly 270 degrees, but to make it such that a full-range turn of the wheel causes the pot to turn IT'S full range, whatever that range might be.