This single story of Africa ultimately comes, I think, from Western literature.
After listing many facts and giving many anecdotes, Dr. Adichie does a good job of reminding her audience of both what she means by and her point about "the single story". She highlights the danger of a closed-off perception while keeping the audience both engaged and feeling trustworthy of her point. However, she does go through a lot of content quite quickly, and by reminding the audience of her focus to her speech regarding "a single story," they are guided in understanding how all the content connects to her point.