I think a lot people saw Kwan's skate and thought she had done enough to win. It had been a long time since a ladies skater who was the favorite went out and pretty much nailed all of her planned jump content. Frank's face at the end of her skate said it all. She went out and planned 7 triples and landed 7 triples. She might have been the first ladies skater to do so at an Olympic games. I think what people didn't expect was for Tara to have the kind of magical, once-in-a-lifetime skate that she had. The conventional wisdom, which lifelong Tara supporters are totally free to gloat about because they never believed the "hype", was that if Kwan skated cleanly and well, she would have won because Tara couldn't compete against her second mark no matter how she skated. Tara proved everybody wrong. Her tech was excellent with a 3/3 and a 3/1/3 (at the end!) and her performance was so on fire and fast and sure that she bridged that presentation gap.
I remember there was an old article that asked like a dozen or so journalists who would win gold before the ladies event, and almost all of them said Kwan save for like two. I also remember Jeni Meno even saying that after Kwan had skated, there was a real feeling that she had done it, she had won. There was also Scott Hamilton's commentary after Kwan's skate. He was commentating as if she had secured gold and how her skating is what the ISU wanted to promote. Tara changed the conversation really quick after her skate.