Welcome to the world of e-books where all standards are really just 'guidelines'
that publishers do
their way. (this applies to ALL metadata fields)
I gave up long ago and hand number all series indexes. (even the Series_NAME is inconsistent)
1) because the metadata is also iffy, if not wrong.
2) there those in-between releases (teasers, deleted scenes) from Author sites
3)watch for series name overlap with books from a different AUTHORS SERIES . I rename the series to: series (author initials) to keep them separate
4) Omnibus editions that need a
range indication: series_name [1.03]