If it works like Google-Search, results are not an estimate, or not even random, it looks like it is inflated .... for example, if you navigate to https://www.google.com/search?q=zzzfgh , it tells you 5,900 results until you reach a page where there is no more results and it tells you 484 results (for example, I never seen an estimate that was lower then the real count)
Duplicate removal generally happens after the initial count is received. That's why you can get major variations in the count and the actual number of results.