That's my problem, Marciano seems to benefit from being given the benefit of the doubt, so to speak.
Frazier proved his mettle against better fighters, and in the process lost against two of the greatest heavyweights ever (beating one of them however). I don't think the latter half justifies ranking him lower.
For arguments sake I would also bring up that Marciano was very nearly (some would say he was) outboxed by Roland La Starza.
I have not seen the fight, I am not saying it should have gone to La Starza, but he is nowhere near the level of the men Frazier has losses to in the book.
I also do not think that fight should take away from his ranking.