The "box model" in CSS works like this:
width + padding + border = actual visible/rendered width of box
and height + padding + border = actual visible/rendered height of box
Box 1: Width = 100% (100% + 4)
Box 2: Width = 460px (460 + 4)
Box 3: Width = 460px + padding of 40px + 4px border (actual width is 504px)