The only way they should decline is if the URLs get indexed.
And I think there are few other ways for this number to decline. Here is my experience:
- If you return 404/410 for "Not selected" page, the number will decline
- If the "Not selected" page gets indexed, the "Not selected" number will decline (as TMS said)
- If you redirect "Not Selected" pages, it will NOT decline
- If you noindex page previously indexed, it will increase
- If you noindex page that is "Not selected", the "Not selected will stay the same
- I am guessing that if the page is blocked by robots, the "Not selected" should decrease
- I am not sure what happens if the page has canonical link element set to point to another page, but if it is treated the same as Redirects, then this will not have an impact on "Not Selected"
I wish google would break "Not Selected" into two buckets, as I would like to see a number for "the page not blocked in robots, not redirecting, not noindexed, does not have canonical pointing elsewhere, but we have not selected it because we do not like it". That would be a really useful figure.