This is probably the effect of the light direction. Polygons that are facing the light are more bright, than those not facing it. You can alter the direction where the light comes from in the preview as well.
I think I know why it happening and it trigger me like a firework.After I textured and exported I relised that my some faces were not flipped as they should be.
After I flipped them the textures turned darker.
So what I think it’s that I have to texture the model again and this will solve the problem