How can two feedstuffs that have the same chemical composition (see below), have different nutritional values?

Question

(chemical composition ie: % water, %ash, %fiber, % protein, % fat, & % carbs

Answer 1

Here's a guess. Some of the components will differ. If the source of the protein is liver, for example, it will be higher in B vitamins than, say, beef or chicken.
And the difference in fats is huge. Vegetable oils, especially olive oil, are beneficial and can lower cholesteral and triglycerides. Animal fats contribute to elevated levels of the same numbers in your body. Trans fats (margarine, shortening) do enormous damage to your arteries.
If the source of carbs is whole grains, it will deliver the nutrients that are missing in refined white flour or white rice.

Hope that was what you were looking for!

Answer 2

Terms like "protein", "fat", and "carbohydrate" are very misleading. There are millions of different types of each, and their nutritional value (or lack of it) varies. You would need to know the precise proteins, fats, and carbohydrates present in order to determine the real nutritional value.

Answer 3

They shouldn't. They probably just have different serving sizes.