[mortehu]

Assuming there has always been 50% light balls and 50% heavy balls, the probability of picking 4/4 heavy balls from the earlier period and 4/4 light balls from the later period by chance is 0.4%.

Although this statistic might be less relevant if the balls were not statistically randomly selected.

[ada1981]

Presumably the eBay balls were probably homers or hit into the stands, no?

[Jackim]

They could also be foul tips or grounders thrown to the crowd.