I just did a quick check with my 5800 levers, and I think I know what's going on. The cable attaches at a point near the front of the lever (the red arrow below) which is indeed 36mm from the pivot, but it is suspended above the plastic for a length over which it can change its angle, so the effective attachment point for purposes of this calculation is the point at which the cable's movement becomes constrained by the inner structure of the lever (the green arrow below) and that's closer to 21mm from the pivot.
