Hello David! Welcome to Q&A, I love your intro!
To answer your question, the results change often for Keyword Difficulty because the algorithm is based on the other sites in the top results that the keyword is competing with. For example, a site other than your site goes down in rankings or Domain Authority, the Keyword Difficulty percentage would change because of this. There are several factors involved in the algorithm we use, but I find this the easiest way to explain it. Does this help?