Confusing Mammogram Recommendations

This article from The Atlantic is the best explanation for what - to me, the casual news consumer - seemed like contradictory statements about revised mammogram guidelines.

I remember feeling like one day I read that the guidelines for routine mammogram screening had changed and then, the next day, I read that although the guidelines had changed, they hadn't. (It turns out, as you'll see below, my memory reversed the order of these events.)

Here is the link to the US Preventative Task Force's Recommendation (USPSTF), which revises the recommended age for routine mammograms from age 40 to age 50, published in the November 17, 2009 Annals of Internal Medicine. The day before, however, The American Cancer Society's Chief Medical Officer released a preemptive statement.

The American Cancer Society continues to recommend annual screening using mammography and clinical breast examination for all women beginning at age 40. Our experts make this recommendation having reviewed virtually all the same data reviewed by the USPSTF, but also additional data that the USPSTF did not consider.

So which is it? What are these "additional data"? And what's the harm of being extra cautious? Questions like these are what I think Crewdon does such a good job of clarifying.