i think theyre meant to show the rate at which the values change on the different kinds of surfaces. on the clay it meanders from dark to light gradually. the plastic has quite a sharp jump from the medium shades to the highlight (it gets light fast where the light is concentrated), but a more gradual descent into the dark tones. The metal has the darkest tones and lightest tones right next to one another sometimes with very little transition, and can jump back and forth in dark and light areas, eg stripes of light, dark, light, hence the squiggly line in the transition graph.