I agree that every draft is unique and the value of a pick, especially in the early rounds, is determined by what players are left on the board and what teams want those players. Still, I can see a team wanting some means of gauging how much certain picks have historically been worth in trade, to use at least as a starting point.
When Jimmy Johnson put his first chart together, he had someone go through past years' trades and use that to mathematically estimate the historical value of each pick -- if pick A was traded for picks B and C, and pick C was traded for picks D, E, and F, then A must be worth... Of course a lot of extrapolating or estimating must have been involved. Since no one has mentioned anybody doing that this time, I guess this new chart is only somebody's best guess based on what seems right?