The earlier 170lb/77kg came from a study of US military personnel many, many years ago.
The main point is that ANY standard weight schedule needs to be based on a sensible statistical study of a population typical for that style of flight .. ie, there is no reason why an operator cannot use several different sets of standard weights in different circumstances.
The Australian regulatory suggested weights are based on a study done by John K quite some years ago and the report is still floating around.
It is a bit useless not to base weights on stats otherwise the outcome is a bit silly at times .. eg a charter for a football team using mums and dads weight schedules.
If the population is representative, the use of standard weights works out quite well ... I can recall, as a young buck on Fokkers, occasionally weighing the punters for a critical takeoff ... invariably the then use of 170lb gave very good results just about every time. ...