Your diffuse value determines how much diffuse light is reflected from your surface.  Nothing in nature reflects light diffusely at 100% with very few exceptions; specific wavelengths are absorbed and the remainder reflected back into the environment.  Darker material will reflect less than lighter.  Hence, setting a diffuse_value of 1 signifies that the texture surface is reflecting 100% of your environmental light, this does not occur naturally.  In addition, the render engine will misinterpret these values and attempt to calculate for a brighter surface.  Diffuse_values should be turned down for all surfaces, including skin textures and especially if you use it on the Poser Surface.