This defensive rating is not very good either, no offense
When you look at it for the NBA it varies greatly from the real net rating and defensive rating that NBA.com uses (which to me is by far the best because it isn't a complex formula, it is simply how good the team is defensively when you are on the floor). Marcus Camby use to compete for the most blocks each year but was actually not a good defender. He was constantly looking for that block and his guy got a lot of easy baskets and dunks and the team as a whole allowed more points when he was on the floor. So according to your defensive rating he was great, according to what actually happened he wasn't.