When aluminum is first cast it goes thru a fluxing process to make the impurites float to the top. They use Chlorine gas and Argon gas in the process. Different allow have higher amounts of impurities the 6262 alloy is the worse. After the flux process the aluminum is cast the metal flows thru a trough and thru a porcelain ceramic filter. Sometime dirt still gets thru and ends up in the finished cast and cause tiny pores or holes in the material. So in essence its a hole caused by dirt. Its not a common thing they maybe have one incident for every million pounds cast.