Well the box method is not supposed to be the exterior dimensions of the car exactly. But you also need to take into consideration the location of the cog (box becomes longer, wider or taller if the cog is further away from the geometric center). And also like phil said how much is the mass centered near the cog or more towards the edges. So the jag has more weight on the outer edges than the v8sc. Thicker steel (maybe), more heavier bodyshell, heavier windows, bonnets, doors etc.