It came up recently that that second site, and other similar ones, scrape only books in links to Amazon, sometimes other sellers. And the sites showing these lists are usually affiliated with Amazon, e.g. hackernewsbooks says:

"The website is sustained via referrals from Amazon book links. ... All links to Amazon, Safaribooks and O'Reilly get extracted once a week from Hacker News posts, make sure they are indeed books and then rank them based on how often they are mentioned and the karma of the user"