"Get Album Artwork" restoring sort information on purchased tracks

When I import a CD and ask iTunes to "Get Album Artwork" and then I sync with my ipod I find that some of the original sort information is restored to some of my music purchased from the iTunes Store (completely separate from what I imported) and it shows up different in my ipod then when I originally modified the information. I have to correct those files every time and every time it is a different bunch of artists/tracks. Why is this? How can I turn this off.