Proteins are measured on a bio absorption index, as compared to the most readily acceptable and absorb-able protein - Egg white.
So when your pet food contains say 40% protein, that may be so, but the key thing to find out is what source of protein the food uses because the body will not absorb all 100% of the available (40%) protein.
depending on the pet food, millions are spent each year in the pet industry to raise the bio absorption of good groups by animals. unfortunately it is hard to classify all pets (say dogs for eg.) into 3 categories - small, medium, large - because their metabolisms differ, and thus the nutrient absorption will differ as well.
Thus the introduction of breed specific foods, and the expansion of the three previous categories of dogs to five, now including toy and giant breed dogs.
Then there is the pitfall that all dogs, even of the same breed are not the same. much like humans, some have faster metabolisms than others and thus are susceptible to different levels of nutrient absorption.