Nothing new... see that more often with google. (10 o's, click next, only 3 o's left).

That's why I use Duck Duck Go these days and only go back to google if I can't find it there (not that often).

It's a side-effect of how Google indexes the web. They have an index for "site:bitcoin.org" and probably an index for "san francisco" (because it's a well-known pair, although possibly this query uses the separate indexes for "san" and "francisco").

Each index lists the web pages that include the term. Google then merges the web pages indexed under those terms, discarding any web page that doesn't satisfy all parts of the search query.

Without checking every candidate web page, Google has no way to know how much overlap there is between web pages matching "site:bitcoin.org" and web pages matching "san francisco". Google doesn't want to check every candidate page just to display the first page of search results, so the count of search results is a very rough estimate. As you move further through the pages of results, Google has the data it needs to refine the count.