This is the link that Sandman posted at 05:21

https://www.youtube.com/watch?v=eKzcIE3M7To

He starts in the Key of F and ends up in the key of G. Check it out. This might have happened if a battery driven recorder had dying batteries but it is not uncommon for unaccompanied singers to drift higher in pitch. It's one of the reasons I never sing without backing. I drift up in pitch.