Roughness is explained well in Is there a way to measure the consonance or dissonance of a chord?
In particular the Plomp-Levelt curve is derived, which has various dips showing how simple intervals (3/2, 4/3 etc) are less rough than the average.
However the curve appears to have no dips for the interval 7/6 and above.
Why is this?
Is it because the curve is empirically derived, and human perception cannot (on average) distinguish these intervals from arbitrary ones?
Or is it a limitation of the model represented by that curve?