revised stories are the same ones you read before, but, well, revised... edited. parts have been rewritten, added, deleted, or otherwise changed. the author will sometimes repost all or part of a story if he or she feels that the changes have been significant enough to merit it.
as for why the changes were made... basically, because the author felt that the story would be better that way. there are tons of possible reasons (information was given to the readers too soon, information that the readers needed was missing, the plot would work better if it was taken in another direction, the author came across a better idea, the author thought of one or more scenes to add, the author wanted to improve style or flow, someone pointed out that something in the plot or characterization didn't make sense, etc) , but they all pretty much boil down to "because i like it better this way."
hope that helps.
Paul
