Why was this point chosen as the origin for the Saturn V coordinate system?

David Hammen gently corrected my erroneous comment on this fine answer to the question: How is the height of a rocket measured? by pointing out the following figure in the Project Apollo Coordinate Systems document.

It shows that the origin of the coordinate system is defined to be 100 inches below the engine gimbal reference plane.

Why was this particular point chosen, instead of, for example, the bottom of the engines? Or the gimbal plane itself?