I was just reading Temujin's thread on Jinyong 'watering down' Jiu Yin in order to make Yang Guo seem more powerful, and I agreed with it; logically speaking, Guo Jing should be WAY above the Greats by that time. Why wasn't he?
Then I got to thinking about the novels, and what they and the characters inside represent.
Guo Jing is obviously a Confucian hero. Very strict, very moral, very upstanding, and very orthodox. The way he learns martial arts is very 'orthodox' as well; he found an extremely orthodox martial arts manuscript, written by an Imperial officer (Huang Shang) who was fighting to rid the land of 'deviant' elements and maintain the Emperor's rule; as close to Confucian martial arts as Confucianism can get.
Yang Guo, in turn, is a very Daoist hero. Rather than conforming to societal norms, he acts as his nature sees fit; unrestrained, uncontrolled, uncontrollable, he truly is Xi Kuang. Even the way he learns martial arts has strong Daoist overtones; eating magical snake gallbladders, being taught by a supremely intelligent condor, and learning the Way of the sword through the writings of a long dead master, on a remote mountain retreat.
Zhang Wuji, in turn, is a Buddhist hero. Forgiveness, clemency, kindness, compassion, those are his keys; even as a child, when his parents committed suicide, even the benevolent Zhang Sanfeng promised to teach Wuji martial arts to avenge his parents, but Wuji "didn't want to kill anyone, (he) just wanted his parents back!" The Jiuyang Shengong he learned, as well, was written by a Buddhist, and was a long lost Shaolin treasure.
Then it hit me. Throughout Chinese history, one of the main points was the "harmony" of the Three Ways; Confucianism, Daoism, Buddhism. Each had their own sphere of influence, and their own use; neither is superior to the other, nor any is inferior to the other. The three are equals.
Perhaps this is why Yang Guo/Guo Jing/Zhang Wuji's relative martial arts levels are so ambiguous in this trilogy; Jinyong intentionally wrote them ambiguously, so that the three would seem the same. In this way, he did not put any of the three Ways which the three Hero's represented over each other; he wrote three wonderful stories, about martial arts being expressed in the three various Ways, making clear that he did not believe Daoism, Buddhism, or Confucianism to be superior to each other.
This, then, is the reason that Yang Guo=Guo Jing=Zhang Wuji, even if purely due to martial arts reasons/storyline reasons, one would imagine differently. Because no one Way is superior to the other two Ways, and so no one of those three fighters, is or can be superior to the other two.