The recent study you mention was a review of 16 studies exploring the link between cervical cancer and IUDs. The studies were from the 1980s and 1990s, and three of them were performed in the United States. Overall, cervical cancer rates were 30 percent lower among women using the device compared to nonusers. Keep in mind that the finding was a correlation and so does not prove that IUDs actually lower the rate of cervical cancer. But the authors surmised that the reason IUDs could potentially reduce the risk of cervical cancer is because insertion of an IUD causes an inflammatory reaction that increases the immune response in the cervix. This could lead to the clearance of the human papilloma virus.
Source : http://beatricedailysun.com/news/local/scrapbook/getting-an-iud-to-reduce-risk-of-cervical-cancer-may/article_62de8bd8-7b04-516a-926e-8b6a303d6a2a.html