Why does the Revit Wire Impedance Factor table have separate sections for Three Phase and Single Phase?

The table published in the documentation provides two sections simply a matter of convenience.  The data in the Single Phase section is the same as that in the Three Phase section, multiplied by the factor 2 / SQRT(3).  As an example, if you look in the first row for #14 in Steel Conduit at a power factor of 60, in the Three Phase section, the value is 3.322 and in the Single Phase section, the value is 3.836.  Multiplying the Three Phase value by the factor yields the value in the Single Phase section: 3.322 * 2 / SQRT(3) = 3.836.