Just checked trip comp and last 75 miles since refuelling have been around town - 20.7 mpg
Could be a variety of things:
tyre selection
tyre pressure selection
innaccuracies of onboard trip computer
speed he drives at
the driving techniques he uses.
I know for a fact the tyres are changed every 12-15k (lease car), he checks pressure every journey, he drives usually at 75MPH, leaves very big gaps between cars infront to avoid braking etc.
Using similar techniques I have acheived over 53 MPG in a Peugeot 106 Gti, and in the same car I have managed a whole 9 MPG (not great for a 1.6 900kg car).
My current weekend toy (Peugeot 205 Cti) ranges from 15MPG on a light thrashing to 40MPG driven carefully at 70MPH.
I would say technique and a well maintained car in evey way can gain you / lose you 10MPG or more. (not saying you or anyone drives badly or doesn't take care of the car!).