There was a note in the DOE paper, and I'm not sure of the entirety of its scope, but they skipped all non-fossil fuel sources in their analysis?
Comment 7: The U.S. Average
Electricity Generation factor (Tg = 0.328)
is based only on fossil fuel generation,
But that leaves out like almost 1/3 of the grid, at least as of 2009
(I'm rooting for the renewable section and gladly give my local windfarm a couple extra bucks)