Why does the A350-900 call out to retard at 20 feet above the runway?
Why is it like this and not like normally at 0 feet?


Normally is not 0 feet: all Airbus (maybe with the exception of the A380, that might have a longer flare) have a "Retard" call at 20ft, not at 0ft. From the A320 Flight Crew Training Manual:


  As a reminder, the ”RETARD” aural alert will sound. In flare, this aural alert will occur at 20 ft, except in the case of autoland, where it occurs at 10 ft.