You keep mentioning the fact that Carpenter "sucked" and "underachieved", which is not true.
He had a 3.45 season ERA, pitched 237.1 innings and really only had one bad month, which was May.
He had ERA's of: 3.89, 3.00, 2.68, 4.08, 2.15 in the other months.
Although September was his best month, he was really good in April and August, and excellent in June and July.