Google has spoken about being on the edge of indexing is similar to Google thinking those pages are on the edge of quality - they seem to go hand in hand. If Google does not deem the page to be high quality enough to be indexed, it won't be indexed. John said the other day that it is common for a page to "pop into the index for a bit, and then pop out again."
John was asked about this Search Console chart that shows URLs being indexed in their respective mediocre positions for a while and then dropping out (still indexed but no impressions):
John replied on Mastodon saying, "That can happen, and probably isn't particularly uncommon. Things can pop into the index for a bit, and then pop out again. When things are on the edge and the system has to make a yes/no decision, sometimes it goes one way for a while, and then the other way again. The web is noisy, I think that's fine."
As a reminder, if you want to improve your site's indexing ratio, you need to improve quality. Google has been saying they do not index everything for years, Google simply won't index all your pages and it is impossible for Google to index everything, Google said.
However, if Google thinks the page is high quality, it will index and rank that page. Being on the edge of that is where you can see the page teeter-totter back and forth.
Forum discussion at Mastodon.
