Songs added through the desktop app don't count track duration the way iTunes does it. Because of this, many local tracks added to playlists through desktop app are not available, even if added to iTunes. I have 450 local tracks. Approximately 40% are unavailable. Because all of them have one second longer duration in iTunes than in Spotify desktop app or in windows explorer although they are exactly the same files. I even wonder how many years you will look for a reason before you come across this topic. 

The screenshot was taken on the desktop app. This is absolutely the same file. But the top one was added to the playlist via iOS app, and the bottom one via desktop app.