The odds usually reflect a bit of both the betting market and the predicted outcome. The bookies have to set a price before they've taken in any money and they often offer crazy prices to attract people to bet on someone i they think they've no chance of winning or example. Say on the day of the fight it came out that Khan had a minor injury but was still going ahead with it, the odds on him would lengthen even i most of the bets were already placed. It's as much about predicting what will be bet as responding to what already has been, i would think.
The bookies set an opening price and made Barerra favourite, since then money has come in for Kahn so now he's favorite....simple as that.
Stop complaining you khan haters!!! if you think that his glass jaw is such a big problem go and bet against him and get realli rich or extremely broke!!!