Related % in the Keyword Magic Tool is based on the URLs being displayed in the top 20 results for the keywords you are analyzing. This metric compares the main searched keyword in SEMrush's first two pages of results (what you searched in the top search bar) to the related keyword's first two pages of results. Depending on how these URLs relate, you will receive your Related % metric.
For example, look at what happens when we search the seed word "movie" in the Related report.
Compared to the related keywords "what are playing in the movies," or "what movies are in the movie theater now" shown in the results, the related % is 30. This means that 30% of the URLs that are displayed in the top 20 organic results for "movie" are also displayed in the top 20 organic results for "what are playing in the movies" and "what movies are in the movie theater now."