More on Manuals vs Automatics

Strange thing - with the 6 speed manual SS, I have to drive very very conservatively to achieve 15 to 15.5 litres per 100 km.  With the 4 speed auto Monaro, I drove it like I stole it, and the worst I could achieve was 15.5.  It was very easy to keep it into the 13's.  Why such a major difference?  Maybe it's diff ratio, maybe it's because the Monaro has 6,000 km on the clock and the SS only has 1600?  Who knows.