We do expect some wavelength dependence of the cross sections (and thus of the wind optical depth), BUT the lines we fit cover only a modest range of wavelengths.And in the case of z Pup, nitrogen overabundance (not in calculation shown at right) could flatten out the wavelength dependence even more.
OR perhaps clumping plays
clumping (alt. “porosity”) certainly could
play a role in the overall reduction of wind optical depth.