... is the different tunings people use and the different keys people like to play in.
I try to practice a lot at home, and that's where the magic happens. We tune to A440, so most of the time it really isn't a problem. However, some songs need those rich, open chords to sound right so we modulate them up a half-step. Playing a 1/2 step off from the recording is really awful sometimes.
Paradise City is particularly grueling. How many of you are in cover bands, and do you tune down or not? Early on, I asked our singer if she wanted us to tune down and she said "absolutely not!".
Why not tune exactly to what the song you're covering is tuned to?