Without wanting to continuing the discussion of merits, Norton and Postlewaithe were only nominated. Kathy Bates imo is like some actors selective in what she appears in, so that is perhaps why she may seem under-rated.
If we take it to the extreme of award merits, why did it take the Oscar Committee so long to give Martin Scorsese an Oscar? One Oscar in all his career; but with a load of classic films (Raging Bull, Taxi Driver, etc). Yet he wins an Oscar for a film that is a rip off of a Japanese thriller called Infernal Affairs.
You could say that Scorsese is the most under-rated director in Hollywood (according to the Oscars), yet he is admired by his colleagues in the industry.