This. The reason why I think Floyd may appear to have such high stamina is because he's inclined to spend the first few rounds more on the defensive side. That is, he focuses more on trying to block and evade his opponent's punches while being more careful and very selective with his. It is his defensive ability to block and avoid punches that affords him to buy the time to study his opponent's attacks, come up with a solution to neutralize/counteract them in the early rounds and then apply that solution in the latter, to which point he has already adapted. In other words, his adaptability and seemingly high stamina stems from his defensive prowess.
Taking advantage of weakness - whether openings to certain punches, reliance on a punch, also he knows when an opponent is trying to take a rest and attacks whenever his opponent is doing this
His ability to stay cool, calm and composed in any situation that has come up. He's been in a few situations that most would panic and lose their cool, yet Floyd always stays on point and never strays away from his game plan. On top of all that the guy can box and has an abundance of natural talent.
His preparation. Floyd is ready for anything that gets thrown at him. Because he is so well prepared and conditioned, he is able to adapt easily and with confidence. I've had big sales meetings before and thought I had prepared for everything only to have the client throw out a wrinkle I wasn't prepared for and damage my confidence. Floyd's the guy who has an answer for everything because he's done his homework.
Its his ability to sell a fight which isnt there, 42 have failed , bull. Only about 10 could have passed. Fight Margarito , Cotto , Williams , Tszyu and Floyd would not be saying '42 have tried and 42 have failed'.