It is all just ridiculously inexact. These are balls made by human beings by hand and by sewing machine. That are composed of animal skin. If you took a case of balls off the production line one day, and another case one week later, put them all at 13.0, subjected all balls to the EXACT same conditions, you could easily see wildly varying results between the 2 cases.
Maybe the first case was stitched together by Helen, whose arthritis was acting up that day. The next one was stitched together by Joe, who has been on the job 2 weeks.