It's the difference between a solid bar and a hollow bar. All they did was do a stress analysis to see what diameter and wall thickness hollow bar would have the same stress as a 1" solid bar to come up with some weight savings.
It's something like Stress = T x C/J, where both C and J have rod radius in them. J, if memory serves me right is to the cube and to subtract out the hollow, it's J1 - J2 or something like that.
I need my Machinery Handbook, here so bear with me. pdq67