|MadSci Network: Physics|
I asked several metallurgists about this and got no definite answer. We did come up with some ideas tho. The first question was is it still so? That there might be some hysteresis in the recovery of the metal. The next question was, Are you sure it's the ball that changed and that something hadn't accidently become deformed. The metal should be examined,I don't know the material,and it is possible that a phase change was induced or the surface became oxidized or cracked thereby slightly increasing the volume. Make some careful measurements and see what evidence appears. My first guess is hysteresis and that they have fixed themselves by now. If not careful measurements and observations are in order.
Try the links in the MadSci Library for more information on Physics.