It's because the balls are old. When they are brand new, most are perfect. They shrink over time. And different parts of the balls shrink differently.
I have a set of new old stock Rachig, which are regarded as some of the best balls ever made. They were made in West Germany. The balls don't look played at all. They all measure about .015" smaller on the diameter, which is a MILE. And in addition to that, each number is sunken in probably another .015". You can see it with your eye and feel it very easily.
Other sets I've measured the black portion of the number is flush with its neighboring surface, but the entire "white part" of the ball that contains the number is at a different height then the rest of the ball. It results in the ball being oval. This is just a very tiny amount, about .0005". Can't be felt. But it shows that the various pieces of the ball change size differently.
My contention is over time, something in the balls shrinks. Maybe some oil or something from the plastic evaporates over the years. I'm not sure what it is. But its more than the common idea that it is wear from sliding on the table.
I personally think more is going on than wear due to friction that results in balls getting smaller.