http://www.defenselink.mil/news/Mar2003/b03112003_bt110-03.html
The Thursday Sidney Morning Herald refers to the bomb's mass as 9545 kg (a conversion that is both excessively precise and inaccurate -- about 20 kg too high), so they are probably assuming that the 21 000 lb figure is the correct one. (It would have been better expressed as "about 9500 kg.")
