Keep in mind it goes further than that. US customary volume units don't match up with British ones.
One British gallon is about 4.5 liters, where a US gallon is about 3.8. Quarts, pints, and cups follow, but fluid ounces are another thing. A US gallon is divided into 128 fl. oz., while a British gallon is 160. This results in a US fluid ounce of about 29.6 ml, vs. 28.4 ml for the British one, and also affects teaspoons and tablespoons.
One British gallon is about 4.5 liters, where a US gallon is about 3.8. Quarts, pints, and cups follow, but fluid ounces are another thing. A US gallon is divided into 128 fl. oz., while a British gallon is 160. This results in a US fluid ounce of about 29.6 ml, vs. 28.4 ml for the British one, and also affects teaspoons and tablespoons.