It's also likely true that a library that bought 10 copies of a book initially is unlikely to buy 10 more copies once there have been 200 circulations and they are needing to be replaced. They may only buy 5 replacement copies since the book is likely to be less popular than at initial release so it will take much longer for the next 100 circulations to occur.

As for anecdota, I have more than once borrowed a library book and then purchased a copy so I could read it again or to finish it if demand is strong enough that I would have to wait weeks or months to be able to borrow it again.