As the Genealogue points out the UPI's headline for an article on Michelle Obama, "Future First Lady had an Illiterate Ancestor" is a little unfair. Every first lady in the history of this country probably had an illiterate ancestor.
I suspect the UPI may have thought "great great grandparent" was closer than many. However, I will point out that several of my paternal great great grandparents would likely have been recorded as illiterate, since they didn't know English. They may have been literate in Yiddish, or other tongues, but not English. My maternal great grandparents from Transylvania were similarly 'illiterate'. That is often the case with first generation immigrants to America, unless they come from an English speaking country.
The original Chicago Tribune story had a more appropriate headline. A first lady closely descended from slaves is newsworthy.