Yes the voltage is relative to ground for the logic state, the differential signaling requires a source of negative voltage and a more complex front-end, being

Yes the voltage is relative to ground for the logic state, the differential signaling requires a source of negative voltage and a more complex front-end, being relative to ground you just need a digital IO operating at 5v or 3.3v and a ground pin, no additional hardware (although current-limiting and pull-down resistors are suggested), as a result it can be fully and easily bit-banged. The twisted pair (where one wire is the bus and the other the ground) is one of the suggested approaches that helps to reduce the interference induced on the wire.