The US units predate Imperial units and are different. Especially in the case of units for volumes. e.g 473ml against 568ml for a "pint".
It's only since WWII that both inches were redefined to be exactly 25.4 mm
It was pity stayed his hand. "Pity I don't have any more bullets," thought Frito. -- _Bored_of_the_Rings_, a Harvard Lampoon parody of Tolkein