After considerable pondering of its purpose I don’t think the scale numbers of #9 thru #20 (3 scales per side) have anything to do with “foot” fractional “inch” equivalents. Meaning, for example on the #9 scale: 2/9th of a foot, 3/9 of a foot, 4/9 of a foot, etc., to 9/9 of a foot @ the physical 12” position is not the way those #’s are intended to be read.
In my opinion, what you have is a per say “grid layout rule(er)” for marking/calculating the position and/or number of “points” per linear foot on a drawing or piece of material.
Like iffen you wanted 14 rivet holes per linear foot you would use the #14 scale. Or iffen you wanted 20 lacing stitches per linear foot you would use the #20 scale. And that is why all the scale “lines” are in alignment with each other at the physical 12” mark.
Just my opinion though.
And ps: a bricklayer's ruler has a "special" grid layout on it also. As does a cow, beef or livestock "weight measuring tape". You don't "weigh" them, you "tape" them to determine their weight.