One thing to keep in mind about the pet food industry is that it is considered "self regulating" meaning there is no one watching what they are doing. There are no regulations concerning their advertizing or just how honest they are about what they put on the label.
The FDA groups pet food in the same category as "feeds." There are some very appalling things that the FDA has approved for use as pet food so take all those claims with a large grain of salt. It takes some time and study to get to the bottom of what is actually in that bag or can of food.
I do believe that it is worth it to buy the best food you can possibly get for your pet but finding what food is actually the best is a tricky task. Most big name companies are some of the worst offenders as far as using low grade ingredients and then adding a ton of synthetic vitamins in order to make it meet the nutritional requirements. You won't find the details of how they manufacture their food on any label or even the website of these companies.
If you are interested in some really well investigated articles on the pet food industry Truth About Pet Food is a very good site to read. Susan Thixton has been investigating the pet food industry for many years now while trying to get them to raise the standards but it is a multimillion dollar industry and a difficult task to get the info out.
Truth about Pet Food