For boxing, I don't think winning every single round would help also. Remember Pacquiao vs Bradley? Pacquiao won every single round but yet, Bradley is determined a winner.
But majority of the time, you're right. Decisions will always favor the champ. Only way to beat them is dominate them every single round like the way Cain Velasquez crushed Junior Dos Santos on their rematch, fight no.2, or knock out the champ like the way marquez knocked out pacquiao or pettis knocked out henderson, or notably, weidman knocked out anderson silva.
Like Poker Orifice mentioned here, it's all about money. If money was placed on the underdog, they'll declare the champ as a winner so that they win all the bets from people who placed their money on the underdog. It's business and politics.