I've done the circumnavigate the globe thing twice. Each time I have bought a series of one-way tickets based upon where I needed to be and when. Never did the trip intentionally, its just that my schedule had me closer to home going West than returning East.

I do know that an actual around the world ticket is controlled differently than most. Last year I had an RT ticket from IAD to SIN & back. The outbound (through NRT) was cancelled and UA had me booked on an outbound through Frankfort then my original return through NRT. When they tried to ticket it the system rejected it and I was told because they were not allowed to convert my ticket to an around the world ticket.