https://www.ala.org/advocacy/bbooks/frequentlychallengedbook...
a list of the Top 10 Most Challenged Books in order to inform the public about censorship in libraries and schools. The lists are based on information from media stories and voluntary reports sent to OIF from communities across the U.S.
So it's (1) voluntarily submitted, like an Internet poll, (2) it is from libraries AND schools (not just school libraries), and (3) the library had to already carry the book, which means that if the library itself felt like the book was too inappropriate to shelve, it will not be represented here.
In other words, take the NPR article with a grain of salt, or skip altogether. Whatever you learn is unlikely to be accurate.