It's just stubborness and pride on the part of the opponents. Guys never want to admit that they faced something they'd never seen before. That's why fighters would get knocked unconsciousness by Tyson and then try to claim they'd been hit harder before.

To given another example, IIRC, Toney claimed that Peter didn't hit as hard as some of his other opponents, which is obviously BS.

I remember after Tito KO'd Joppy, Joppy admitted that it was the hardest he'd ever been hit, and I remember being surprised because fighters are never that honest.

As other people have mentioned, fighters do the same thing with speed too. They always try to say that they've fought someone who's faster than their current opponent. I remember when Greg Haugen fought Pernell Whitaker, Haugen said that he wouldn't have any problems with Pernell's speed, because he had already fought Vinny Pazienza, and Pazienza was faster than Pernell. That was pretty funny.