It just happens. Everything you buy from jars of peanutbutter to cans of soup is allowed a degree of "other" substances.

I was at frys some time ago. I like to buy the sushi sometimes. It's the handmade type that is made inhouse. I got it home and opened it and there within the white stick rice was a black beatle bug. I picked out the bug and ate my sushi.

I shop at Trader Joes. They have some of the finest food substances offered. This often comes with more apperent violations that are not as well hidden else where.