After some quick reading it seems unlikely that the bowser should be inaccurate, they get checked regularly by the operators and regulators and I imagine the penalties would be steep.
http://www.caltex.com.au/LatestNews/....aspx?ID=13454
If you think the servo's pump is inaccurate you can report it.
http://www.measurement.gov.au/Pages/...the-Truth.aspx
Expansion space is apparently minimum 5%, so a tank designed to hold (say) 70L must do so at a maximum 95% capacity and will actually be a 78.95L (or more) tank.
http://mechanics.stackexchange.com/q...les-vent-trick