ok thank I found on Nvidia tutorial a answer to explain the focus:
"When mipmapping is enabled, both OpenGL and Direct3D support a texture mapping feature known as texture level-of-detail (LOD) bias. Texture LOD bias can be useful to avoid unnaturally crisp reflections. Modify one of this chapter's examples to provide a positive bias for the cube map texture used as the environment map. This creates blurry reflections."
but I don't know if threejs include this ?