Cardio is a strange thing...I can spar and roll for a decent amount of time, but when the fights on and the adrenaline is raging I gas much quicker. Pro fighters (with the exception of Lesnar and Carwin), learn how to keep a cool head and pace their adrenaline.
Its all about the pacing. I've gone the distance in ammy MMA (9 minutes) a few times and felt like I could do another 3 rounds. But I've also felt like I was gonna pass out after 2 minutes of the first round in a different fight. Shit, just wrestling with a D1 guy for a few minutes has me feeling like I ran a marathon.