I hadn't really given it much thought before this. It is annoying when the arm rests are not quite in the right position, but I would hope that Lexus would get something like that right.
The only thing that worries me slightly is the fact that there is the mouse controller thingy. They may have made the arm rest shorter in order to have people's hand match up with the controller, which could possibly have made the arm rest shorter. Although it should be that your hand does match up with the mouse so hopefully they get the right balance between everything.