Boxers chins + KO's.

Why is it that a lot of boxing fans jump to conclusions over who is capable, and who is not capable of knocking out a certain boxer in the ring based on previous events?

The reason I am asking this is because I have seen a number of predictions recently on the upcoming Mayweather Mosley bout. And well the majority of the predictions seem to point towards a Mayweather SD or UD, or a Mosley KO (I am one of the people who thinks Mayweather is going to take it by decision).
But lets say I posted a comment on a random Mayweather Mosley prediction thread and maybe said Mayweather is going to knock out Mosley in the middle rounds (and I have seen this comment on a few occasions now). No doubt I would get a lot of replys stating Mayweather couldn't knock out Marquez who came up from his natural weight of 135/140lbs, so there is no chance Floyd can knock out Shane.

The question I would put to these people is, How do you know? Have you not seen stranger things happen in a boxing ring?...Have you not seen a boxer knocked out before by a boxer not considered to be a power puncher?...but then steps in the ring with a power puncher and takes his punches all night long? :huh

 

I should point out that I am only using the Mayweather Mosley bout as an example as it is the biggest upcoming bout.
The point I was trying to make is, a lot of people on this forum are unaware, or choose to ignore the randomness of the sport (but like Bandido has said, you can't calculate randomness, it's all about odds and probabilities). Whether this is influenced by there favourite boxer, or there dislike of another boxer. I could make a prediction for a bout that boxer A will knock out boxer B...and obviously there are people who are going to disagree, fair enough. But then you get people who will laugh at your prediction and question your knowledge of the sport.