I think I understand what @DarkSir23 is saying. If I download metadata for a book that already has the series name and number, and the metadata found matches the series name, but not the number, the series info does not get updated. If the series name is different, the series info is updated.
I did this with both the Amazon and the Goodreads metadata source.
As an example using just the Amazon metadata source enabled for the book
https://www.amazon.co.uk/dp/B00873GRU4.
Firstly I downloaded the metadata and the series is "Wool Trilogy" and the index is 1. I changed the index to 2 and used the bulk metadata download to get the metadata. When I reviewed the metadata, it had nothing in the series field in the left side, and "Wool Trilogy [2]" on the right.
If I change the series name to something else, the metadata review shows the downloaded series name and index on the left.