Saitama is allowed in those meetings because several S-class heroes become his best palls, not because his powers are accurately known to the association. Furthermore, we know for fact that several S-class heroes were given rank based on merit or perceived merit alone, without ever taking the test (King is the obvious example, because if they had adequately tested him, he wouldn't be a hero) . Which again highlights how, if they'd correctly extrapolated his strength, he wouldn't be a low rank.

All of those others things I already noted & discussed about.