why is US gallon different from imperial gallon?

Answer 1

The US gallon is about 3.8 liters where the Imperial gallon is about 20% larger (4.56 liters). As for why it's different one is a US measure and one is British. They are just set up by different people just like why is a yard different from a meter. Does it make a lot of sense that we are using a different system of weight, length, volume? No, but that's the way it is.

Answer 2

http://en.wikipedia.org/wiki/Comparison_of_the_Imperial_and_U.S._customary_systems

this link gives history