Imagine this: most MMORPGs consist of multiple virtual servers, which are all instances of the same world but with different players. They are copies of each other.
Now, using the same content & design, what if different servers proposed different worlds ? The cities, the NPCs, the monsters, the items, would all be the same.. but spawned at different locations. The world map would be different. A city that was in the west coast on server A could be located on the east coast on server B. The cities/dungeons would have exactly the same design, they'd just be located at different places.