Yeah but, the US customary system has one set of units for fluids and another set for dry goods. The imperial system has only one set defined independently of, and subdivided differently from, its US counterparts.
The Winchester measure was made obsolete in the British Empire but remains in use in the US. The American colonists adopted a system based on the 231-cubic-inch wine gallon for all fluid purposes. This became the US fluid gallon.
Note that one
avoirdupois ounce of water has an approximate volume of one imperial fluid ounce at 62 °F (16.67 °C). This convenient fluid-ounce-to-avoirdupois-ounce relation does not exist in the US system. So whilst the imperial gallon, quart, pint and gill are about 20% larger than their US fluid measure counterparts, the fluid ounce is about 4% smaller.
Many Canadian brewers package beer in 12-imperial-fluid-ounce bottles, which are 341 mL each. American brewers package their beer in 12-US-fluid-ounce bottle, which are 355 mL each. This results in the Canadian bottles being labelled as 11.5 fl oz in US units when imported into the United States. Because Canadian beer bottles predate the adoption of the Metric System in that country, they are still sold and labelled in Canada as 341 mL. Canned beer in Canada is sold and labelled in 355 mL cans, and when exported to the US are labelled as 12 fl oz.
then there's Celsius, centigrade and Fahrenheit. And don't even get me started on the English language. Even our meters... metres? are different....
just sayin'.