I did notice a decline in BJ's guard between round 1 and 2. In round 1 it was way high up on GSP's back, to the point I was worried about a submission. After that round his guard was much lower. It didn't look like he was trying to get it up there and failing, but rather he settled for the lower guard because he was tired.
Watching Penn on his back reminded me a lot of my BJJ instructor. He's a really little guy, no more than 5'5" 165lb. But he's soooo incredibly flexible it's insane. I never came close to submitting him, or even being competitive. Every time I got him on his back, he would squeeze that guard shut tight, and it was impossible to break. After that it was only a matter of time before he would get me in a triangle or something.