That's kind of like saying its weird and unbelievable that most animals on Earth has four limbs and an internal skeleton. There are exceptions sure, cephalopods and insects but reptiles, mammals, birds and amphibians all have four limbs and internal skeletons. Once life on a planet finds something that works, it sticks with it. If mandibles had evolved early enough in the development of life on Planet Predator than yeah, they're probably pretty common on that planet.