Media Gallery

Why is it that the default media gallery does not see images that are already uploaded in WordPress? Example. I use media gallery to upload a photo or image to WordPress on my external site. Later I decided to install a local version on my Mac using Mamp. I installed the WordPress exactly like I have it on the live site and I even copied the SQL database from the live site and placed it on my computer changing the URL paths for WordPress to match the local settings. The photos were no longer listed in media gallery. I had to reupload them to show up again. So I ended up with duplicate photos or images on my system. I think Media Gallery needs to be fixed so that will see the photos or images that are already uploaded on the system.