rear inner fender question

I am currently replacing the trunk floor in my 65 holiday and while siting in the hole where the floor pan used to be cleaning up the remainder of the trunk floor I noticed that the driver's side inner rear fender is a different shape than the passenger side. The passenger side is much more rounded (arched) starting at the floor where as the driver's side comes straight up for close to a foot before arching...it also has a large hollow flange, for lack of a better word, on the top...anybody know why the difference?
Just curious...never noticed it before...