Well each list reports differently. For example the New York Times uses reporting bookstores, meaning certain stores (around 30) spread throughout the country. These stores report into the Times with their most successful titles for that week. USA Today is based on sales as is the Wall Street Journal.
While no one knows the secret to hitting a list, there’s a metric involved in this process. The metric is this: books are sold into stores by publishers early on, months in advance. The publisher starts building interest for this title via its sales team and also something called the announced first print (which is often higher than the actual print run). Publishing is about perception, so the first piece of this is the perceived momentum that a publisher is putting behind a title which will encourage bookstores to order it. The second piece to this is having enough copies on hand to surge the list. How many copies? The average changes because the amount of books published but historically it’s been around 30,000. Then comes the magic word: availability. Sometimes self-published or small press authors will associate an Amazon listing with availability. Amazon is neither an indication of availability or distribution. Yes, you should have a book listing on all the online store sites but a listing and distribution are two very different things. So advanced sales, print run, and distribution all factor heavily into a book surging a list. There are, however, always exceptions to this rule. If a book surges suddenly and in a short period of time it can hit a list. Last year I was having lunch with a publisher who said a book they were working on hit the top 10 of the New York Times with little marketing and only a 4,000 print run. How did this happen? The author had done some of their own online marketing and the viral factor kicked in, sending people into bookstores, and it surged up the list.
— Penny Sansevireri of Author Marketing Experts

for some reason this reminds me of those things in high school where the students vote for homecoming king/queen or “best body” or whatever. i had a girlfriend who was in charge of tallying votes for one of those once for a school with a few hundred kids in the class and she said the winners only had like 4 or 5 votes apiece. there were so many people to vote for you only needed to garner a handful of voters to win. in regards to books, if there were a way to get a couple thousand people to buy your book in a week — even if that’s all you ever sell — that could be enough to make you a “bestselling” author.
You are correct, sir. That’s what some people call velocity. Many authors encourage their readers to buy their book the very first week so that it will get as many sales as possible and possibly make it reach the bestseller charts. Of course, if you knew each and every bookstore in the country that reports their sales to the NYT, then, theoretically, you could make sure all your books are bought just at those places and, who knows, it would be a bestseller even if it didn’t sell elsewhere. The best way to tell if a book is a true bestseller is by the USA Today and WSJ, as noted above, because they use the actual numbers reported all over the country via Bookscan.