One thing people often overlook is that Cardio levels are often very different from fighter to fighter/ person to person no matter how hard they train... Even if they both do the exact same work...
Everyone has a ceiling and limitations... Things like lung capacity and various other elements play into it.... Even if you trained as much as you possibly could that doesnt mean that you are going to necessarily have be able to do what guys like Cain, GSP, Diaz, etc. can do.
Just like speed or power. It can be improved, but only so much.
Im not sure if that is the case with shogun... But we hear a lot about it with him and guys like Penn as well.