Many people seem to be under the impression that if a fighter wins a rematch, then a robbery, a mistaken close decision or a poor showing in a previous fight against the same opponent, will just vanish into thin air.
Take for example these fights:
Pacquiao-Bradley: Total robbery. 99% of people think so, except the judges in the fight and a few people who know nothing about boxing or are deluding themselves due to hatred or bias.
BOTTOM LINE: If Bradley defeats Pacquiao in any later fight(s), THAT WILL NOT ERASE THE ROBBERY that occurred in the 1st fight.
Mayweather-Castillo 1: Mistaken close decision. Castillo out-punched Mayweather by a clear margin, in total punches and by a huge margin in power punches, and he had also a higher accuracy ratio. He was the defending champion, and yet he was the one forcing the fight, being the aggressor all fight.
BOTTOM LINE: Mayweather's clear win in the second fight of the two, DOESN'T ERASE THE FACT THAT HE LOST THE FIRST ONE, and he undeservingly "won" a title, due to a bunch of judges that were either sold-out or just biased in favor of Floyd.
Pacquiao-Morales 1: Judges were right in this one, giving the decision to Morales. A relatively poor showing by Pacquiao (compared to the other two fights where he destroyed Morales).
BOTTOM LINE: Yes, Pacquiao won two of their three fights in the end, BUT THAT DOESN'T ERASE THE FACT THAT HE CLEARLY LOST TO MORALES in the first one.
Like these three examples, there are many others, such as Frazier-Ali, etc. etc.
When somebody loses a fight FOR REAL, independently of whatever the judges rule in their scorecards, that big spot will ALWAYS BE THERE in their record, and their legacy as a fighter. And that will always mean that he wasn't really the better fighter of the two, because he lost at least once to his opponent.
If for example Bradley and Mayweather retire "undefeated", everybody will know that their record is fake, because there is the big spot of those two fights that were ruled wrongly, so it doesn't matter if there were rematches where they won and even destroyed the opponent who previously defeated them for real and in the judges' scorecards, or the one who defeated them for real but was wronged by the judges' decision. A loss is a loss, whether it is officially decreed or not, and it will always tarnish the record of an otherwise "spotless" career.