[mormegil]

And possibly not even assigned at all. I looked at the lowest known ISBNs for Czech publishers and a different color stood out: no, https://books.google.cz/books?vid=ISBN9788000000015&redir_es... is not a correct ISBN, I'd say :-) (But I don't know if the book includes such obviously-fake ISBN, or the error is just in Google Books data.)

[Finnucane]

Publishers buy blocks of isbns based on expected need, how the actually assign them may be arbitrary.