"In order to upload your music quickly, Google Play matches your music files with copies on our server. Occassionally we might match with the wrong song. Report the problem here and the song will be corrected to use your copy instead of the matched copy," mentions Google's dialog.
If you click "fix match", you'll get this message: "Please make sure that the computer where you keep this song is running Music Manager and is connected to the internet. The song will be uploaded again within the next day."
Google's copies have a bit rate of 320kbps, so many of your songs will be "upgraded". You can check this by clicking "edit info".