Here's the scenario – you are mixing your track to a stereo medium. At the end of your track you have a nice fade. Just the right length – not too short, not too long. And you end the mix immediately the moment the fade has disappeared into inaudibility. Perfect.
Four years later, a newer, more high-definition audio medium is on sale to the public. And your master is set for re-release as part of a compilation collection.
But the problem is that your recording is not such high definition as this new medium. When the last moments of your fade come to an end, they don't fade into silence – they fade to the level of the background noise of your stereo medium. In the higher-resolution compilation, there is a sudden and audible change from your not-so-silent-silence to pure, perfect silence.
The mastering engineer doesn't like that so he fades your fade just a little bit earlier than you would have liked.
To be honest, if you are working with digital media this is a very fine point, and only just audible in the worst case. But if you had mixed to analog tape, and many top producers and engineers still do, the transition between the end of a fade that sounds silent in analog terms, and true digital silence, is very plain.
In fact you will hear this in CD's made from analog masters. Either you will hear the sudden cut off when the analog tape ends and transitions to the leader tape, or the fade at the end of a track will be faster than it was on vinyl. Once you have noticed this, it will become annoying.
Actually there is another good technical reason why even your digital recordings should be ten seconds longer. Another time…