Metadata for eBooks, journals, and other content may change. COUNTER reports are always generated with the most recent set of metadata for each item and COUNTER data is never changed after it has been generated. Therefore an item (a book, journal, article, etc.) may have different metadata for different reporting periods.
Example:
Year of publication for a book is changed from 2018 to 2019 on 1 November 2019. The book has been available online since 1 October 2019.
If you download separate monthly reports for October 2019 and November 2019 the reports will have different ISBNs for the book.
If you download one report for the period October 2019 and November 2019, that report will contain double lines for the book – one set of lines with counts for October 2019, and another set of lines for November 2019.