I had a search saved from some months ago, went to check it for some references and got new hits involving local organizations that had nothing to do with the search. Opening up a private browser I see that the searches aren’t 100% matching up either.
my guess is this is just variation in the search algorithms over time. excerpt from their privacy policy above which I think can explain most of the variability you mentioned:
“For example, we may know that we got a lot of searches for “cute cat pictures” today, but we don’t know who actually performed those searches. That is, viewing search results on DuckDuckGo is anonymous. And we only save these anonymous search queries — completely disconnected from any unique identifiers like IP addresses — for just enough time to analyze anonymous trends like popular searches, so that we can better serve you. For local search results in particular, we’ve further engineered a solution to shield your precise location from us and our content providers that sends us a random location nearish to you, which we also never log to disk.”
So they do use your geo IP and popular searches to weigh results which can change over time.