system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does it mean to be a New York Times Bestselling author? Why does it seem like every book uses this title?
gpt
There are about a dozen different categories, and a new list is created every week.  That means it is possible for there to be over 500 unique bestsellers each year.  In addition, there are six or seven categories with monthly lists for 70ish more possibilities.

Also, publishers design their marketing strategies around making bestsellers.  If they have two books in the same category, they might adjust the release dates so they get two one-week bestsellers instead of one two-week bestseller.