And you can't exclude the possibility that the person just lies about the hearing ability and cheats.
Audiophiles the world over are claiming that differences stem from USB cables, green rocks placed at random in the room, etc. etc. This is nothing new. Our last resort used to be to tell them that they wouldn't be able to pick the difference apart in an ABX test. In the past this would lead to denial of the utility of ABX tests but recently I've seen a disturbing tendency for audiophiles to simply produce impossible ABX results leaving us gibbering "but... But that's impossible! Proper ABX tests have shown these differences to be inaudible! You must have done something wrong! " Problem is, WHAT? What are they doing wrong?
Let's start with codec ABX testd conducted using the foobar2000 ABX plugin. Just today I read someone claim scoring 87% out of 30 trials of 320kbps mp3s vs FLAC encoded from the same source (of unspecified music). Now I wouldn't be so incredulous if this were a known golden ear listening to fatboy or something but this is just some random person listening to presumably random music. So what do you reckon he screwed up?
Almost every time I thought I heard a compression artifact when listening casually (LAME V0), it turned-out that the original CD had the same "defect".