I recently installed a k&n drop in air filter. Upon inspection of the air box I found that there is a carbon type looking filter built into the lid of the airbox. I also observed that the factory air filter has an additional layer of material like a 1/2 in of poly Filter matting. At 15,000 miles this poly filter matting was extremely dirty. I also removed the additional carbon type filter built into the lid of the airbox. Has anyone else had the same experience and do you know why there is additional filtering materials?