People read for reasons beyond pleasure. I might be doing a study or comparing authors, I might be reporting on history or researching someone. I might be doing a comparative analysis.

Ten checkouts seems low enough that it's likely to get regularly cause problems. 100 checkouts seems too high. So start bisecting. 50? Almost certainly too high. 25 maybe reasonable.

They also might have p90 checkout rates and found that 25 is a data driven choice. Librarians are smart people.