Commercial DVDs and scanning



I have been using Nero DVD speed to scan the DVDs that I have burned for a while now to help me find the best media for my drives.

Just out of curiosity I have scanned a few commercially produced ( pressed ?) DVDs and found that some of the older ones ( scratched etc) produce very ugly scans.

Does anyone know if Nero DVD speed is designed to give valid results on cemmercial DVDs or is it just for burned DVDs