yet you still call your antiquated system …
… of inches, yards, pounds and whatnot as “Imperial measurements”
No, we call our system of measurements “US customary”. The Imperial system of measurements was a replacement for the traditional English and Scottish units in the UK and its possessions; the Imperial system was not adopted in the US.
Why the hell is the gallon different?
The US customary gallon (3.785411784 l = 231 cubic inches) was originally the pre-Imperial English wine gallon. (There were also ale gallons, corn gallons, and coal gallons in the pre-Imperial English system.)
The Imperial gallon (4.54609 l) is closer in size to the pre-Imperial English ale gallon (4.621152048 l = 282 cubic inches). The Imperial gallon’s size was not based on a set number of cubic inches, but (originally) on the volume of 10 pounds of distilled water weighed in air with brass weights at a barometric pressure of 30 inches of mercury and at a temperature of 62 °F.
any others?
Of course! Both gallons contain eight pints, but for liquids, the US customary pint contains 16 US customary fluid ounces, and the Imperial pint contains 20 Imperial fluid ounces; thus, the Imperial fluid ounce is slightly smaller than the US customary fluid ounce.
There’s another unit called the bushel, which is a larger unit of volume. The Imperial bushel contains eight Imperial gallons. There are two US customary bushels: one for liquids (eight US customary gallons), and one for dry goods such as grains (approximately eight English corn gallons).
The US still uses both avoirdupois pounds and Troy pounds for measuring mass; Troy weight is used mostly for precious metals. The Imperial system has only retained Troy ounces (but they are identical in both systems).
The US used to define certain SI units in terms of US customary units. This changed in 1959, when the US customary units were redefined in terms of SI units. However, the “survey foot” is still based on the pre-1959 US definition of the meter in terms of the yard. This wasn’t redefined because of the amount of survey data that was based on the survey foot.