The notches aren't the same distance though...
I don't know if the student is taught, or is expected to figure out a way to "commonize" different length "wholes."
One of the "wholes" is a number line carved into 6ths, the other (a different "whole" length) is carved into 8ths. The student should know that a direct comparison won't work, because the "wholes" are not the same length. Either one line has to be stretched, or the other shrunk. That right triangle figure that includes two "wholes" is one way to accomplish this equalization.
If I was teaching this to my kids, I'd describe the projection as a shadow. The shorter line has to be "closer to the light" in order to make the sames size shadow as the longer line. The comparison of is made at the "common length shadow," not at the line.