Why does the copy sound better than the original

Good question, and I'm no expert on that one!
It would seem the practical course of action is to email either Peter McGrath or Robert Harley, and find out if there's been an exploration of why this happens, along with whatever limitations (exclusions) they've come across.

There was an issue in TAS where Peter McGrath, engineer for Wilson recordings, heard a copy of the CD and it sounded better. I believe the word he uttered was "impossible!"

As it turned out (on CD at least), the CD-R/RW pits are wider than the commercial CD pits, and so the copy is cleaner, and sounds better than the original. Robert Harley also commented on this phenomenon in the pages of TAS 3 or 4 years ago when it was discovered that it was the blank discs that were responsible for the better sound.

It sounds as though you know how to do this, Geoff. I do not. How about doing it as you have the knowledge and filling us in on what you heard?

eoff:
I'm not interested in doing the experiment. You are. Therefore, my suggestion was that YOU do it, not direct me to do it. I'm simply REPORTING what Peter McGrath and Robert Harley posited. It was THEY who posited that the larger pits allowed more information to be recovered more cleanly, much in the way that Classic Records put entire symphonies on 45 rpms, instead of 33, because, they said, they grooves in a 45 were larger and thefore, it improved the sonics.
People offer theories all the time. It's how life advances.
I see no need in trying to prove or disprove someone else's speculation, if they are the experts. That's why I made it clear that this point of view came from others, specifically named them, and where the information could be found (TAS). If you write TAS, or Peter McGrath, I'm sure they'll be happy to open a dialogue about it.