A fair map in TN is 6R-3D.
Actually not. TN is R+14, or in a 50-50 national race the state should split 64% R to 36% D. Studies have shown that for every point that a state swings from 50% the delegation swings by 2%, so that in a 50-50 national race the TN delegation would be expected to be 78% to 22%. With 9 seats that corresponds almost exactly to a 7R - 2D delegation.
One recent paper that cites a number of the studies is Gerrymandering or geography? How Democrats won the popular vote but lost the Congress in 2012, N. Goedert (Research and Politics 2014). The particular 2 to 1 effect is well documented in studies, some going back to Tufte in 1973. My metrics of skew and polarization mirror Tufte's goals of plans being unbiased and responsive.
Can this be explained by statistical modeling, and do the underlying assumptions of the model hold true for all places and all times?
