Every engine ever built burns a MASS of fuel when mixed with a MASS of air.
LPG weighs less than petrol.. ie .. the same volume has less MASS so you get fewer miles per VOLUME.
If we bought fuel by MASS ... ie kilogrammes... the fuel consumption in "miles per kilo" (or pound if you are pre-metric) would be identical.
We have a MAF (MASS air flow) regulator to measure the MASS of air entering the engine .. it does NOT measure the volume of air. The MASS of a fixed volume changes with temperature and pressure, your engine produces more power on a dry, cold day than it does on a hot, wet one, as dry, cold air is the more dense .. so is a higher MASS, thus more fuel is added to burn that extra air .. so you get more power ....

HTH
EDIT .. before any pedants decide to join in .. I base the above on the calorific values of the fuels as being almost equal ... petrol having a range of 44.8-46.9 MJ/kg and LPG having a range of 44.1-46.1 MJ/kg. Those figures will vary depending which web site you look up .....
