There are two "Standards" for any gauge. First is the mathamatical equation for the given gauge which in the case of the 16ga is .662" or 16.8mm. The 2nd "Standard is whatever suited the maker at that particular point in time. Much variation will be found between makers or even the same makers over time.
L C Smith at one point in time specified .650" for their 16ga bores, but later went to the normal .662". Manufacturing tolerances will of course give slight variations to even the specified sizes.
