Hi!
Thank you very much for the proper details, it helps me lot.
The problem is, that there are other tags on that particular post as well, so the plugin is pulling anything it can from each of them. I think, it would be better to have at least one field to be used for keyword matching.
I noticed, that you use Yoast seo, which has a focus keyword field, which I think is perfect for this purpose. I have made the following change: https://i.imgur.com/At3h0ub.png
Now, the plugin will use this keyword field, and hopefully give you much closer matches based on the focus keyword.
You can also define the items to be shown for each post, if you want to be more specific about the related items.