I don't know. How's that for an answer for all of these guys who know everything, and all of their answers are different.
Balls today are made of plastic, therefore the weight can be made closer to specs.
Back in the days of wooden bats and iron men ball were cork center and twine. It was harder to control the weight between balls.
In 1967 ball weight was 6.0 min to 6 3/4 max. In 1968 the weight was moved up to 7 1/4 min to 7 3/4 max.
Since then came the poly balls and the metal & com bats and I really ever herd of any "Stop the presses" anouncements about ball weight.
It was big news then because it took 15 to 30 feet off of a long hit.