In my view there are two possible explanations for this finding. One is that many cancers resolve spontaneously, that is, the body's immune system is able to detect and eradicate the abnormal cells. This is the conclusion that the articles suggest. The other possibility is that the radiation of the mammogram itself is causing more cancers.
The study is called The Natural History of Invasive Breast Cancers Detected by Screening Mammography by
Per-Henrik Zahl, MD, PhD; Jan Mæhlen, MD, PhD; H. Gilbert Welch, MD, MPH. The women studied live in four Norwegian countries in 1996-2001. The 4-year cumulative incidence of invasive breast cancer for the two groups was 1268 vs 810 per 100 000 population; relative rate, 1.57; 95% confidence interval, 1.44-1.70. Even after prevalence screening in controls, the cumulative incidence of invasive breast cancer remained 22% higher in the screened group, and the higher incidence in screened women was observed at every year of age.
Arch Intern Med. 2008;168(21):2311-2316.