snidely.too wrote, on Mon, 06 Oct 2014 12:22:31 -0700:

A draftsman's spline is a flexible ruler, originally formed
(AIUI) by shaving down a standard wooden spline until it was
thin enough to be flexible.

That explains why a woodworking spline, which is straight, would
be applied to a draftsman's spline (which is decidedly curved).