thor - if you wonder how 800 came to be then consider the following:
Density of liquid methane == 600 kg/m3
Molecular weight of methane ==18 kg/kmol
1 Sm3 (15 deg C/1 atm)==0.042 mol
=> 1 m3 liq methane = 600/18/0.042 ==800 Sm3
Standar m3 is actually just another measure of moles
Best regards
Morten