GB:
It's because this debate, like the previous two, are being widely reported as a win for Obama. It is also being widely reported that McCain was snide and disrespectful. That's the kind of stuff that makes up peoples' minds, not the debates themselves. Also, the fact checking sites aren't really painting the picture that McCain is truthful and Obama is not. I don't belive most voters think for themselves, I believe they act on what they are told to think.
Debates aside, I have long believed that Obama will win simply on novelty, because he's black and because kids think he's cool. That's good for 20 million votes right there. Combine that with a country that is disgusted with the way the Republicans wasted the contract with America, and it should be a landslide.