Could be using a step down transformer to get 110 from 220. Otherwise, if using one leg of the 220, then it would be one leg relative to neutral. Ground is suppose to connect to the frame. Neutral is the center tap. Both ground and neutral are at the same electrical voltage (usually called zero). In older wiring, having a single wire serve as both ground and neutral was common for 220V equipment such as ranges.
The problem with using a common ground + neutral comes in if you think about cutting that tab off of the plug, or any other interruption between that wire and the fuse box. If that happens, then the "grounded" frame of the machine is not actually grounded, but is wired through the motor, etc, to one or the other of the two 110V legs. That can be deadly, as touching the frame could mean you are the ground/neutral path.