He maintained his energy with Pavlik because he did not have to work hard with Pavlik. He had everything down against Pavlik and fought a totally relaxed fight, which is much less tiring.

Against Joe, Joe kept him thinking the whole fight and tense even when Joe was not landing anything. Joe kept him on edge and Bernard wore himself out.

What I said above may be mostly BS, but I think it at least contributed some. I also think Hopkins may have trained better for this one, was in better shape etc. It is hard to say.