Makes perfect sense, actually. If your opponent lands 5 big power punches and those still don't knock you out, why should your opponent get rewarded for that round? In the judges' eyes, a punch is a punch, regardless how powerful it looked.
Sure, if you look at GSP's face, you'd think he should have lost that fight. But his face has always looked battered and beat up, post fights. He probably has thin skin tissues which causes bruising/cuts a lot easier. In matter of fact he has looked a lot worse than that, before (see his post fight interviews when he fought Condit and Shields).