Why is it always that a fighter showed up for his "last" fight, but not for "this" fi

Isn't it possible that the same determined, powerful, fast, well conditioned Mosley that fought Margarito was the same that fought Mayweather? Vs Margarito, he was much faster, could land at will, could see everything coming. Vs Floyd he just couldn't land anymore than round 2. And you tire the more you miss (DLH vs Floyd) Does anyone really believe that had Floyd and Mosley fought January 09, the outcome would have been different? Styles make fights and I think 10/10, prime for prime, Floyd would have always beaten Mosley.