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I've heard that Google will process queries that don't seem to get results for the searcher and they improve if you come back to it. I never test this because I've usually given up (wasn't important) or moved on to other ways to find the information.

Not sure about Google but DDG changes the results when I come back to them. So, for example, if you search for something, click on one of the links to open it and then go back to the search results, there will be a couple of new ones added.

Not sure if some are removed also but it definitely changes.

In a ironic twist, it now returns this HN page.

Google has multiple search clusters, and indecies are not fully in sync, since they are updated asynchronously. So often the difference is the cluster your query is landing on.