Answer:
US Gallons are different than imperial gallons.
1 Imperial gallon = 4.54609188 litres
1 US gallon = 3.78541178 litres
There are approx. 3.785 liters (US) to a gallon.
There are approx. 4.546 litres (UK) to a gallon.
Note that a liter is a metric measure and is the same worldwide.