How are initial weights/relevancy scores determined?

I’m preparing a presentation about how site search works on a site I maintain, and how posts are ranked, and for that purpose I am doing some research on Relevanssi, since this is the plugin we use for search.

I’m a bit puzzled as to how the relevancy score of a post is calculated. I’ve created a test post with a word that is unique for the entire site and then searched for that word. I’ve echoed $post->relevancy_score in the search results to experiment with how I can influence the score by modifying the post.

A few odd things that I’ve noticed, and for which I didn’t find any info in the knowledge base:

• When I mention the word once in the body of the article, and search for it, the score of the post is 52.29. Where does this number come from?
• When I modify the post, without changing the frequency of the word (moving it about, putting it at the top or bottom of the body, etc.), the relevancy score increases by varying amounts. This happens each time I save the post. The increase is not constant, but varies between 1 and 1.6, approximately. I’d love to know why this is.
• Is there any way of accessing/displaying the inverse document frequency of a term?

https://wordpress.org/plugins/relevanssi/