When I previously worked for a power sports manufacturer, we would have tooling for only a couple of different spline profiles in an effort in minimize cost and increase commonality. The spline profile stayed the same regardless of shaft diameter. So the spline count was strictly a result of how the profile fit onto a particular shaft diameter. I.E. a 1" shaft may have 20 splines, and a 1.25" shaft may have 25. It had nothing to do with dividing 360 degs by an even number. I can't say if that was the rational behind Ford's design of axle splines, but I have been in engineering long enough to know we generally all think alike.
