[Magick-bugs] -modulate changes color?!

[Ilya Zakharevich]

# ImageMagick pixel enumeration: 1,1,255,rgb
0,0: (238,241,246)  #EEF1F6  rgb(238,241,246)

I do
  convert a.txt -modulate 80 b.txt

The result is

# ImageMagick pixel enumeration: 1,1,255,rgb
0,0: (175,189,212)  #AFBDD4  rgb(175,189,212)

How comes?  I expect all the values to be multiplied by 0.8...

    Version: ImageMagick 6.5.0-0 2009-03-15 Q16

Puzzled,
Ilya