I'm far from thinking it is bug but I just don't understand this behaviour.
Since it is my first try with such simple "assumed_gamma" I post it here. Here
is description of my problem (I play with POV 3.5).

I have designed sky. It takes some time to render so I decided to create
enviroment map (is it well named?) and use it in second image. So I've added
spherical camera and traced image to 5000x5000 PNG. Then I have used 'Basic
scene' template from Insert menu. I've removed all objects, light, camera was
replaced and sky_sphere was used my PNG as image_map. But render looks lighter.
I figured out there were assumed_gamma in this template equal 1.0. I hadn't
assumed_gamma in my original scene so I checked in the documentation what is
default value of it. In 6.11 contains: 'assumed_gamma : Display_Gamma'. So I
checked what is Display_Gamma. 5.2.2.2.1 : 'If the Display_Gamma keyword does
not appear in the INI file, POV-Ray assumes that the display gamma is 2.2'. And
2.2 was reported by output of renderer. So to duplicate effect I wrote into new
scenefile global_settings{assumed_gamma 2.2} but render was still wrong. Then I
completly removed assumed_gamma from script and render was ok even if renderer
reported 2.2 in output during render.
--
#declare _=function(a,b,x){((a^2)+(b^2))^.5-x}#default {pigment{color rgb 1}}
union{plane{y,-3}plane{-x,-3}finish{reflection 1 ambient 0}}isosurface{ //ABX
function{_(x-2,y,1)|_((x+y)*.7,z,.1)|_((x+y+2)*.7,z,.1)|_(x/2+y*.8+1.5,z,.1)}
contained_by{box{<0,-3,-.1>,<3,0,.1>}}translate z*15finish{ambient 1}}//POV35

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