Some Grayed Out Songs Still Playable?

[tmanbez]

I noticed a song that I had "Saved" months ago has become "unavailable" or grayed-out.  I can comprehend why this happens to some music on Spotify due to licensing agreements and whatnot... but what has me curious is the fact that the song is actually still playable when you select the "Play" button when hovering over the album, happen to hit the song when in shuffle mode on a playlist that the track was added to, and other various circumstances.

Is this because I had the song saved to my library prior to it becoming unavailable / grayed-out?  The scenario is rather interesting and I haven't been able to replicate this with other unavaile tracks (such as Dr. Dre's albumed titled, "The Chronic").