The Wikipedia site explains that a PRT unit should weigh no more than 600 pounds and carry three or less people. The unneeded cars just stay parked until someone requests one. Since the pods don't travel unless in use or when being sent to the busy section during peak hours they average a higher average occupancy than busses. These things combine to make it twice as efficient as a bus running at full capacity.
Busses often run almost empty in the off peak hours which makes them very inefficient. Surely if everyone rode mass transit instead of using cars the busses would be more efficient, but that won't happen (in the USA).