Simple, because they named "their" cartridge in the old European fashion not for the bullet, but for the bore diameter, sans rifling grooves. Remember, Their "9mm Makarov" pistol cartridge uses .365" bullets, so it is actually a 9.3mm too. Or, the European "6.5mm" numbers use .264" = 6.7mm, the "7mm"s .284" = 7.2mm, the "8mmS" .323" = 8.2mm bullets. The maximum length of the 9.3x53R Finnish (CIP name)is 53.3mm, so "53" is a more apt designation.