be vs have been

It was a small room on the ground floor with a tiny window under the stairway. The furniture could not have been much simpler.
Czemu w 2 zdaniu jest could not have been a nie could not be much simpler. Jaka jest różnicy między have been a be? Przecież "could" wskazuje na czas przeszły

could be - nie bylo takiej mozliwosci, np. nie mieli pieniedzy
could not have been - figura retoryczna - 'byla najprostsza z mozliwych'