The first really usable line in my Ar-Ne lamp spectrum is at 4158.590A. Below that the lines are either blended or too faint unless I overexpose the red end. Including the 3946.097A line gives a considerably worse fit at the blue end. So the accuracy of my wavelength calibration drops off below 4000A and I normally don’t report my spectra below 3900A.
The ISIS wavelength fitting report issue is curious. Here is a report I got recently.
The lambda values listed are all exactly the values I provide in my .lst file. Maybe it depends on the spectral calibration mode you use?