tin/nickel is a coating, but a conductive one, you cannot have a conductive path, but an electromagnetic path. the varnish acts as an insulator so that the entire ball of copper may be used. Look at all the provisos under table 8. It is really a VERY general table, and due to change from manufacturer to manufacturer. Remember the first article of the code (OK up until 2002) "This code is not to be used as a design or specification guide" that's why.
