One thing I'm testing now is to see if it makes a differences if I offline music directly using the Google music application from my android phone.  This of course means that I can only download music by setting either artists or albums to offline.  This afternoon, I also moved over individual songs to my phone by creating new playlists for them on Google music web.  I then set those playlists offline.  The next time Google Music redownloads offline music files.  I would like to be able to see if the redownloaded tracks are from the playlists only or not. 

@Traxter I agree with you that the 1000 song limit for playlists is annoying.  Although for me, this wouldn't be an issue if Google just allowed for individual songs to be selected for offline directly within the Google music android app.  I don't understand why this song level selection for offline is not possible from within the android app.