I don't profess to know the answer to this as I don't have enough information about the shader setups involved, but I'd like to suggest a possibility.

In older shaders, and even some new ones, the same texture is used for both diffuse colour and bump map. Now, the Poser texture manager can only have a single gamma setting for each texture, so one or the other (diffuse / bump) will have the wrong setting. Now, in such cases EZSkin resets the gamma settings for you, and although that still means that one of them will be wrong, EZSkin ensures that the diffuse colour is the one with the correct setting. perhaps it is correcting things for the colour map at the expense of the the bump map (which tends to be less noticeable, in my opinion)?

Free stuff @ https://poser.cobrablade.net/