Rcoky had the heart and the stamina, boxing isn't just about who could hit harder or punch more, no one ever went the distance with Creed it wasn't till Rocky did, Creed couldn't handle the 15th round both times, he had stamina problems, and had never had to go the distance before.
it's like a 100 metre runner who is faster then a 200 metre runner only he can't win over 200 metres it happens.
Actually, in real life, rocky would've won the minute he knocked out Apollo with the last punch. They didn't have to count out rocky, cause rocky slipped from exhaustion. It was only Apollo that needed to get up before the count, even though he was well ahead on points.
Creed fought a dumb fight in the final rounds, as he was killin' Rocky prior.
rocky hit a lot harder than apollo. apollo was seen though more as the older experienced actual "boxer" who was a lot more polished and had better skill than rocky hence he teaching him what he knew later on.
though look at IV, even before the drago and balboa fight, they refer at the press conference about rocky's known punching power.