thanks....But how light gets affected by this?
That is where the Normal part comes in, normals are what decide how specular a material is in model form. The maps take this data and use it to create specular data that the higher polygon models have and transfer it onto the lower models.
Lower res maps will have flatter surfaces through lower amounts of data and the loss of flat reflections seen in the upgrade are purely because these maps now have far superiour detail showing more scratches or holes etc.
Here is an example, my own work.
This first picture is the gun with no normal map. Look closely at the ivory handle, the texture is pretty flat, like the original Skyrim ones.
http://smg.photobucket.com/albums/v690/pinkypics/?action=view¤t=ColtRender1.jpg
This picture shows the same gun with a normal map applied.
http://smg.photobucket.com/albums/v690/pinkypics/?action=view¤t=ColtRender1-1.png
Although exaggerated by going from zero map to actually having a map, it is the same as having a poor one and going to an upgraded version.