I have been posting lots of children's books lately and have found that even though they are WL and have ISBNs (mostly Dr. Suess and beginning reader series), many of them do not match what is listed by PBS.  Publisher, year published and everything else match, but if the ISBN matches  the type of book will say paperback even though the books are hardcover.  OR the ISBN comes up as unknown!

Sometimes publishers used the same isbn # for both paperback and hardback editions of the same book. And sometimes book club editions had a different isbn # than the regular edition.