You can have a song that was released on an artist album, and the same song released as a single (I'm talking about CDs in this example). These different versions of the same song can differ in overall length by seconds. It's the same for compilation album versions and instrumental versions. Some even sporadically go out-of-sync throughout the song.
Is it a product of the mastering process? Is it a product of the recording device? Or something else?
Over the years I have noticed this on a few different songs. One example I can think of off the top of my head is Tupac's song 'Hail Mary'. The album version, the single version and the instrumental version all fall out of sync and even seem to sporadically speed up and slow down.