I with @DNSB In that THE AUTHOR view prevails. (sometimes Amazon does not even include the series, or the give it a different name

)
Some release shorts (could be deleted scenes) thru other channels. Those get fractional numbers as to where the Author says they fit.
Prequels get 0.x numbering.
A stand beside book in the series get a 0
Omnibus editions, I number Start.end (3.07). x.10 fails in Calibre, but usually I number a short as x.5, so it should be easy to twig on the placement