Bernita suggested something that might explain why some authors, like myself, enjoy creating whole fantasy worlds for their characters to play in while other authors prefer to use the existing world of earth as a backdrop upon which to paint their tales. She suggested that it was possibly to do with a kind of inductive/deductive difference in the authors.
Playing off this, I think, perhaps, that some writers like to start with the unreal and work their way down to the real. They begin with an alien planet, say, and then begin to try and build reality into it by creating rivers, and cities, and deserts, and people, all of whom must have strong elements of realism if they are to work.
Other writers start with the real and work their way up to the unreal. They begin with rivers and cities and so on that already exist, then began to alter these toward the fictional, and they create characters who move within these worlds who have never existed.
I still don't know what it is in writers that make them prefer one approach to the other. Probably I never will. Although the difference is endlessly fascinating to me.