There was a difference in the "quality" of japanese steel and US steel in the 30's. The US and other european steels are just "better". The US 16 really had the power of the Japanese 18. The japanese had to go to the 18 because they could not make a 16" gun that would take pressures so they got there with a larger bore and weight. One reason the two ships, really three the carrier shinano was converted from a yamoto class ship that was not finished had to be so big and heavy, to give them the same punch as the american and british ships.
Fun fact the 18" guns also had an AA role. Basically a big shotgun, but reports from american pilots say it was not too much to worry about, but it sure was pretty to look at.
