JPEG quality factor algorithm

Hello,
 
I would like to know whether the algorithm to determine the JPEG quality of
an image (
http://trac.imagemagick.org/browser/ImageMagick/trunk/coders/jpeg.c , line
683) was taken from some papers or other references or whether it was
developped internally by one of the imagemagick developers (ex: by trying to
reverse the quantization tables in libjpeg) ?
 
This algorithm works perfectly for all images created with libjpeg but it
does not work well with images saved from Photoshop ( an image saved with
low quality, say less then 60, will be found to have a quality factor of 90
and above). I have an example of an image saved with 6/12 in photoshop and
imagemagick reports 91%. Did anybody experiement with other encoders then
libjpeg?
 
Thank you!
 
Gilbert