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As far as I know, our algorithm for keyword difficulty has not been changed in any serious way. However, we did just release a fresh link index update, and since the difficulty is calculated based on a number of different link metrics, the scores that you're seeing could conceivably be differing due to these link metrics having been recalculated. I'm honestly not sure what KC is, so I can't really speak to that, but perhaps someone else can swoop in and cover me on that one