Part of the reason for the difference could be that a sky_sphere doesn't 
have a surface. When radiosity hits the real sphere, depending on your 
settings it may make another level of recursion, firing more rays from 
that point on the sphere. With the sky_sphere, which is just a fancy 
background feature, there isn't an intersection point, so it stops.

Also, radiosity calculations are distance dependant, as I recall...and 
since the sky_sphere is just a background, it's distance is "infinite", 
while your sphere is obviously finite. MegaPOV radiosity can compensate 
automatically(which didn't happen with the old version of radiosity), 
but this might trick it into doing something wrong.
Giving your sphere a diffuse of 0(to tell radiosity to stop recursing) 
and making it extremely big(to minimize the effect on distance 
calculations) might get rid of some the difference.

BTW, when using solid colors, why not just use the background feature? 
The sky_sphere feature is mainly only useful for applying patterned 
pigments to the background...

-- 
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/

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