A guitar is never in tune.

The pitch of each string is determined by the tension, but plucking the string stretches it and increases the tension, so raising the pitch of the note. As the sound dies, the pitch falls, so that it's only in tune when the sound has stopped. You can watch this happen on a good electronic tuner. The same applies to pianos, so that an “ff” section will be sharper than a “pp” section.

This is why certain pitch-accurate synthesized instruments sound subtly different to the live version.

But you probably knew this anyway.

ROG.