You would be surprised to see how many books have the wrong ISBN or have a mismatch between the barcode and the ISBN. Not as much of a problem with major publishers now, but some from the 80s and 90s were hilarious.

The ISBN mappings were maybe fifty percent reliable, service by service. So you'd do three or four ISBN lookups and paw through the results, and then failover to the LOC. The Library of Congress numbers were more accurate, but more effort to enter. Nothing worked 100%, and the failure rate on some of the more obscure books was very high.