Google posted in the data anomalies page and on Twitter that it has updated the coverage report within Google Search Console and as a result, you may see more indexed pages being reported as of December 15, 2019. In short, some pages that were "Crawled - currently not indexed" are now known to be indexed.

Here is what this site's report looks like, notice the spike in indexed pages on December 15th (click to enlarge):

Google wrote "The Index Coverage report can now more accurately report on indexed pages. Because of this, some pages that were Crawled - currently not indexed are now known to be indexed. As a result, you may see a transfer of pages from Excluded to Valid state. This does not reflect any changes in your site, but a more accurate accounting system."

From Dec 15, the Index Coverage report can more accurately report on indexed pages. Because of this, some pages that were "Crawled - currently not indexed" are now known to be indexed. You'll see a chart notification in this report. pic.twitter.com/RLgmo6x8jX — Google Webmasters (@googlewmc) December 18, 2019

It makes you wonder - I mean - here, I'll quote AJ:

So ... the report has been wrong for quite some time. Is it correct now? Or is it just less wrong? #seo https://t.co/UmjEPi42NF — AJ Kohn (@ajkohn) December 18, 2019

In any event - if you see the change and your clients go - what is up - you can show them this.

Here is what others in the SEO community are seeing:

Here you have :) pic.twitter.com/LgZPei6VlE — Aleyda Solis (@aleyda) December 18, 2019

Not in all project pic.twitter.com/iVdRFowKiC — Heba Said (@HebaSaidSEO) December 18, 2019

Yes, visible for all projects pic.twitter.com/qFcoOdj6qX — Ramesh Singh (@ramesh_s_bisht) December 18, 2019

Biggest jump was on a sub-domain with chronic XML sitemap issues, so "crawled - currently not indexed are now known to be indexed" would make sense: it saw a jump of over a 1.2M added to valid. pic.twitter.com/MzM5gMJjrg — Jason Lax (@jasonlax) December 19, 2019

Here's what the change in the Coverage report can look like. A drop in "crawled, not indexed" with a corresponding increase in pages indexed. Great to see that Google can now more accurately report on indexed pages. That's clearly a good thing. :) @googlewmc pic.twitter.com/yWWVDQEMMq — Glenn Gabe (@glenngabe) December 18, 2019

Forum discussion at Twitter.