There is some truth to what MJ79 said. Some people, even though they have plastic laid out exactly the same as their reactive balls, just release plastic differently.
That being said, if you measure your PAP on two different strength reactive balls - you can get a different reading on your PAP due to the flare of the core. One thing I've heard some pro's say is if you're wanting to drill up a new ball, take a PAP measurement off of a similar strength ball (if you have one)simply because of the potential PAP difference.