Why would Poultry Meal be the main ingredient in Purina One's "Salmon & Tuna" cat food?

You would think Salmon or Tuna would be first on the list of ingredients to use, but no, both are only 6th. Also, what's with the use of Corn Gluten Meal and Animal Digest? I didn't think byproducts of corn production and unspecified-animal-leftover soup were things we were supposed to be feeding our pets.