I have seen this in antique clay balls....
They used to recycle old clay balls in two ways. They would take smaller "parlor table" sized balls and use them as cores, or they would grind up the balls and bond them together to make a speckled ball. When making speckled balls they just used more ground up 5 ball to make an orange 5 ball, etc. They made bad runs and they also took old balls in on trade so I am sure they used both stocks to remake cheap balls.
Brunswick at one time made a type of clay ball called "compo-ivory" that had a stamp inside the box stating that the balls were NOT guaranteed. If you tapped a compo ivory ball it sounded hollow, that's because it is a thin shell like your ball, coating an inner ball. I despise bar table cue balls made of that crappy material and I always wondered why they would make an inferior product- I guess we now know why. Shame on you, Valley. I might as well buy Sportcraft.