I was wondering how the differences in imperial and US gallons specifically came to be. And also how was the relationship between the US and England play a part in the distinction between the pair?
In the late 18th century, there were three different "gallons" in common usage, the corn gallon (4.4 liters), the wine gallon (3.8 liters) and the ale gallon (4.6 liters).
Britain standardized weights and measures in 1824.
They used a close approximation of the ale gallon. They changed it a little so that 1 gallon of water weighed 10 pounds.
America, meanwhile, had standardized (not by law but by practice) around the wine gallon.
Thus the British got a larger pint of beer than the Americans.