It is a known issue that there are sometimes discrepancies between total search results returned by a search directly in PubMed vs. the PubMed search in DistillerSR's Find and Import References screen.
This is due to PubMed's API updates - the required updates are expected to be made later this year.
To remedy this issue until the update is made, and avoid missing any references, we recommend performing your search in PubMed and exporting your results as the format "PMID". This will create a text file with all the PubMed IDs of the references in your search. These IDs can be copied and pasted into the DistillerSR search field and will return the same results as in PubMed.