I guess it is as the other guy said, the S class either were unconcious (tatsu, darkshine, puripuri, flashy, bang)All that fighting with the Monster Garou and all the hero association that could give Saitama is a mere "A class Rank 39"?
Surely those S class jerks did not spoke any to the association about Saitama's contribution aren't they? He should be above "S class" if not, atleast an S class level title.