I suspect that this is just what happens when you put your potato farm right next to a golf pitching range (or have a disgruntled neighbor who chips golf balls into your potato patch). If you've ever seen mechanized potato harvesters, they aren't too different from golf ball pickers (http://www.rangeservant.us/golf-ball-pickers) except perhaps in size.

I suppose the secret is to have an extra level of quality control for removing the small, hard white "potatoes" with uniform dimples. You'd think they'd do this for rocks and other random debris too, but obviously not?