The Best Match feature in PubMed uses machine learning to help you find highly relevant results, quickly.  Because the Best Match results are calculated differently from the default PubMed search, your total search results may change when you sort by "Best Match."
We developed the Best Match feature to improve upon, but meet the same needs as, the “Relevance” sort order. Best Match is available via the “Sort By” menu, to make it easier to find for users who are familiar with the “Relevance” sort.
Because we designed Best Match to rank by relevance, we expect that you will be most satisfied with the items at the top of your results. As you page deeper through your results, the items should be less relevant. For this reason, we limit the best matching results to 10,000 citations.
Best Match is not designed for comprehensive or systematic searches. To see more than the 10,000 most relevant results of a search, or to use the default PubMed search translation and mapping, we recommend that you do not use Best Match, and sort instead by “Most Recent”, “Publication Date” or another sort order.

For more information on Best Match, please see the following article in the NLM Technical Bulletin:

For more details on how Best Match retrieves and ranks results, please see the following information from PubMed Help: