I'm confused, the title says paperback, but the statistics are mentioning hardcover books. I imagine they sell quite differently.
Edit: Ah, read the amazon quote for more accurate information.
Last July we announced that Kindle books had passed hardcovers and predicted that Kindle would surpass paperbacks in the second quarter of this year, so this milestone has come even sooner than we expected – and it’s on top of continued growth in paperback sales.
Also in the announcement it said that the Kindle books excluded free ones, impressive.