Why is There a Difference Between Wavelength Values Listed in a WDM Analysis and Wavelength Values Acquired Manually on the AQ6331 Optical Spectrum Analyzer?

 The WDM Analysis calculates a central wavelength by a threshold method.

 

A peak wavelength is acquired by a central wavelength between wavelengths, where the level is at -3 dB points on both side of the peak. Therefore, the resulting wavelengths acquired through WDM analysis will be different then wavelengths acquired by normal peak search. 
  

If the optical spectrum is an ideal normal distribution (the spectrum is left-right symmetry), the result is the same as a peak wavelength.