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DaltonST - you are assuming that all metadata is known at the time a book is added to a library, with fiction that's probably true, but with non-fiction less so. Especially with subject tags.
Imagine a passing reference in a biography of famous person A to person B who later becomes also (in)famous. FX: I wonder how many Hollywood bios have a passing mention of HW, someone might have a "Mentioned" column they want to update.
Or an obscure financial instrument confected in the 1970s that becomes very relevant 30 years later. Who knew the role that CoCo bonds would play on Wall Street in the 2007/8 crash, or what a CDS was before the Greek debt crisis struck.
A full text X1 search for 'Coco Bond', 'Contingent Convertible Bond' or 'Enhanced Capital Note' on one of my libraries spat out a results list of ~450 documents - thanks to one of my favourite plugins I was able to tag them with 'CoCo Bonds' in the blink of an eye
BR