While this is certainly one way of looking at things...
I would like to point out something about this chart. Most of the Purina products are given Fs. Many of the Nutro products are given As.
You might recall the pet food recall of 2007. I certainly do, my cat is in renal failure from it. Purina, with all their many many lines of food, had less recalls (and they were with their Alpo product, a pretty low end one that I wouldn't feed anyway) while Nutro, had many more. True, the recalls affected wet food (as opposed to the kibble version being graded here) but do you really want to buy stuff from a company that put melamine in alot of its products?
compare:
Pet food recall