I believe from what I could tell the only balls that have a sort of memory effect are those that are fairly perfect. As in the covering has no rips. So I imagine after a match or two all the balls that had a memory effect will lose this. This effect is caused by the lack of air being able to return in to the ball. I think.
What does “memory effect” mean?
Assuming by memory effect, you’re referring to the tendency to return to an 8" sphere, I found that the balls all have a memory effect, but depending on the deformation it may take longer to return to the original shape. I don’t know how verifiable that claim is, but it’s based on my having stepped on a ball (not straight down) and its taking some 30 or 40 seconds to return to a sphere.
I think what he means is that when you squeeze the ball, a memory effect is remaining deformed and slowly expanding back to the original shape.
This video shows the difference between balls with and without memory at about 5:15 :
http://www.youtube.com/watch?v=twhL0Yk9IMg&feature=related