Bruh, the merit system requires others to give an honest account of what happened, and since most of the S-class were either unconscious, or are have too large of an ego to admit he did that much, so they gave a half-assed version of what actually happened to save face. We can probably honestly thank Zombie-Man for Saitama's rise in rank at all since he was the most conscious and humble out of the people there. They probably also blamed him for Garou getting away