The ingredient list was very similar, the calorie count per cup was very similar so it wasn't like we could feed less.
There is also a difference in the quality and/or source of the ingredients, even if they are labeled the same. It isn't really regulated very well, and "lamb meal" can mean two completely different parts of the animal depending upon the source and the manufacturer of the food. One may be highly nutritious, highly absorbable, while the other is not. So the dog would need to consume more in order to meet its nutritional needs.
But I'm not telling you to change your dog's food. If you're happy feeding Kirkland's Best, or Trader Joe's, or Pedigree, or IAMS, then that is obviously your decision. It's not the one that I would make in my house but you're not me.
I too feel that I am trying to feed my dog to the same standards that I feed my family to. I guess I see some of these foods as more of a white bread and canned soup sort of a diet. That's not how we eat. There are high quality white breads and canned soups, but to me it's not the same as whole grain only etc.