I blame the Americans who started the rot by describing fasteners by spanner size - eg 1/2" A/F which is normally for a 5/16" diameter bolt - rather than everyone else who uses the threaded siameter eg metric and BSF / Cycle etc. That chart from Ron is quite helpful. For some reason the Japanese manufacturers use different head sizes to the standard ISO ones - 12mm as opposed to 13mm for 8mm bolt diameters.
The Americans in their usual fashion, refer to their sizes as "standard".....