Every manufacturer does also make different quality levels of discs. Since Apple is such a quality focused company, I feel quite sure that their discs are the top-of-the-line media being produced by whomever they are sourcing them from. An example of this would be how most set-top DVD players, up until recently, could play commercial VCDs but not burned ones. Why, because the commercial ones were mastered onto higher quality/higher reflectivity discs than were available for consumer burning. So, when you are buying in volume, you definitely can getter better quality media made for you even than the same manufacturer is making available under their own name.
Anyway, I think most people would agree that most of the time you get what you pay for. And one last example, Apple computers have a much lower average failure rate and mean-time between failure rate than other computers. They really are very quality focused.