the brightness depends on the used material and on your light map.

i think with my example material every light map value above 128 brightens the texture and every value below 128 darkens the texture.

<edit> i checked your models and the reason was that they didn't have the same scale. the vertices must have exactly the same positions otherwise the triangles won't be found on the second model. if a triangle doesn't get found its texture coordinates of the second uv-set get set to 0. </edit>