It is not a perfect world. Sometimes the Molten Aluminum has little pockets of super hot gas that just like it there. So they just hang out, flow along, get mixed up in the metal and decide to create a void during the cooling phase. I believe the term is "Outgassing". It happens. As long as they are not huge they should not cause problems. Only problem is they create pockets for Foreign Objects to hide and escape to damage tranny guts once assembled!
