when someone comes over your place to listen to your system, and they bring along their own optical disc media for digital music, some stupid, funny thing can happen...
Your guest plays his one of his CD, which is a recording that is familiar to you, and which you also own.
During playback, you mostly stand/sit on one side, (so that your guest gets to sit at the sweet spot) so you don't immediately notice the problem of central imaging skewing towards the left . But you do notice that somehow the overall sound is slightly sluggish and veiled, and there is some funny "pressure" in one of your ears. The more system-proud and inquisitive host may walk around to the back of the guest and try to "visually" listen for anomalies by checking the soundstaging and imaging aspects of the sound, and that's when you notice that the main vocal image is sitting not true-center, but rather skewed towards the left, so that the image is now situated between the the left-channel loudspeaker and the center point of the left-right soundstage. You scratch your head and ask if your guest is ok if you stop this play and then play the CD (of the same recording) that you have owned for months and have often played it.
Your own CD turns out ok, and the main vocal imaging is now sitting true-center. And not only that, you find that the overall sound not sluggish and veiled like the CD of your guest's, but is clean, dynamic and robust. Soundstage is opened and imaging is clearly delineated and clearly located within their own space in the soundscape. You check your friend's CD and both yours and his are perfectly identical, and you know for sure that there is only 1 pressing that exist (an upcoming, not so well-known artiste from an obscure audiophile label), so there should be no reason why the 2 discs should sound so different.
Your nonchalant guest have no idea what the fuss you are talking about, and he remarks that he takes good care of all his CD discs, and just before coming to your place, he had made sure that his CD discs are all cleaned and he uses a brand new microfibre cloth to wipe them clean of all smudges and fingerprints that might have accumulated despite his upmost care in handling them.