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Hi there! That's because they aren't based on  search volume. Keyword Difficulty takes into account the Page Authority (PA) and Domain Authority (DA) scores of the sites ranking on the first page of Google for the given query. We also modify intelligently for the projected click-through-rate of a given page. This puts more weight on higher-ranking, more visible pages and less on lower-ranking, less visible pages. The formula also accounts for newer pages on powerful domains that may have DA scores but have not yet been assigned PA values. You can read more about the Keyword Explorer Metrics in our Guide. I hope this helps to clarify - if there's anything else I can help you with, feel free to reach out!