Welcome to Android Central! Where did you download these songs from? Your Google Play Music website on your computer will only show your music library that you've saved to the cloud (i.e., any tracks you purchased from the Play Store, tracks you ripped from a CD or downloaded from the web and then uploaded to the GPM cloud, or -- if you subscribe to GPM's unlimited plan -- any track from Google's huge library you saved to your own personal library). If you downloaded songs from some source other than Google Play on your phone, those won't automatically sync to your Play Music library in the cloud.