I forgot about XZ vs DCQ, I was thinking about him against JMZ. Yes, I don't get at all why he was fighting him for so long. It doesn't make any sense.
And yes there are heaps of reasons why people underperform, but we are trying to gauge their actual level so it'd be more helpful when there are no outside variables such as eagles, Huang Rong, etc. With Ci'en JY did not mention any distractions and it still took a day -- which I take it to mean he was very close in level, and improved his martial arts as GWM did.
Against Ah Er, if I remember correctly, they matched palms maybe two three times and then Wuji destroyed him. I don't see any inconsistency there. Against Ah Da, he was fighting an expert martial artist specializing in swords, armed with the Heaven Sword while he never used swords before. I think that's enough to warrant him taking a bit longer to defeat him, and even then it was rather quick and he had no chance of losing. It was just a matter of how many minutes it would take.
The XZ example does indeed have me stumped though.