Another thing to keep in mind is that being named to the Pro Bowl is about that one single year, not a career. Just because an average player like Derek Anderson went to the Pro Bowl, that does not mean he did not deserve that honor. In Anderson's case, his performance in 2007 merited him a trip to the Pro Bowl; each of his other seasons merited him a clipboard on the sidelines. There was nothing faulty with his being named to the Pro Bowl that year.
The other thing is that there is (or should be) a huge difference between being voted to the Pro Bowl and going as an alternate. Unfortunately the two usually get lumped in together, so the person with the most votes at his position (e.g., Manning or Brady) doesn't get much more recognition than the guy that goes as the 8th alternate (e.g., Garrard).
If you want to look for a better distinction find who was voted in as a Pro Bowl starter; or for who was named as an All-Pro as opposed to a Pro Bowler.