Yeah well you have, you just didnt know what it was you were looking at. Monitors are made to comform to how human perception is sensitive. So they have in general more colors in the bright part of the viewport, a mac targets to have a gamma of 1.8 and a windows targets a gamma of 2.2. However this said it dont need to be what your monitor exhibits, the only way to know for sure is to use a tristhimulus colorimeter to verify (also this si the ONLY way you can even start talking of colors with other users since otherwise what you see is NEVER what they see) as the results vary widely between 2 monitors.
So 0.2+0.2 is NOT 0.4 on any normal computers color arithmetic theese systems but rather the value is somewhere around 0.6 - 0.66 wich is about 50% brighter than youd expect it to be! Now because calculating in this scale is near impossible to do right on sample level, and because its inpractical, the renderer assumes you will correct it later. (well it does not it just assumes calculation is linear)
This does not come up in renderings where theres no global lighting because theres no compound interest to boost the color. But when there is each surface acts as a light you get slightly too bright (for dark colors) or slightly too dark (for bright colors)
So unless you are accounting for this it will be off. (if you dont know what it is its safe to assume your not accounting for it)
