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As Effect Digital mentioned, this happens when Google is trying to figure out which page best satisfies the intent behind the search. I would advise checking the rest of the pages that are ranking for the term and see what kind of content these pages have and what intent are they satisfying. You can then see which of the pages on your website are in line with that intent and adjust your on-page SEO elements accordingly. I  would also recommend looking into the backlinks pointing to these pages and see which one has the most links in place and take that into consideration when you decide to make any changes.