I've been wondering about those rules, too. Frank Atienza appeared in three episodes of Hawaii Five-O but is only credited for two: Savage Sunday and Pig in a Blanket. He was also in Leopard on the Rock and he had lines in that episode. So why isn't he credited for that?
That's Al Eben! Wish we saw more of him in Five-O. Loved his character of Doc Bergman!