Because the values are not luminance values. They are transparency factors.

But Poser was not set up to make that decision on its own. I dont' know why. It is left to you to figure out which maps are transmaps. This seems wildly silly to me but it did make the coding and testing easier.

Luminance values (what are in the RGB values of things we look at) are not linear. Rendering with render GC tells Poser you want to convert everything to linear first. This means that all values less than 1 become lower, because they were gamma corrected in the first place so they look right. This is defined by the sRGB standard - that the response curve of pixels on your screen is nonlinear. There are lots of good reasons for this, mostly having to do with two things: how humans perceive luminance, and how computers encode luminance using only 256 possible values. If we strictly dealt with 256 linear values, many of the higher ones would "look" the same to us, and the fine gradations in the lower ones would look crappy. By "bending" the response curve, it creates a more useful distribution of luminance across the 256 possible values.

But - height maps (bump and displacement), normal maps (which are actually vectors), and transparency maps are not colors. They are numbers just as they are. They are not "sRGB" values. They should not be interpreted the same as color maps are.

So we need to prevent the anti-gamma correction that is applied to image maps, but specifying that they already have a gamma = 1.

There is a script in the scripts menu that does this in just a few clicks for all trans+disp+bump maps across everything in the scene. Use it. It's in Material Mods/change gamma or something like that.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)