Thanks for checking the yarpp_related_cache table. You said that the values there look good, but can you verify that the results you're getting (which you describe as random) are not what are in the cache? Similarly, if you refresh a page, do the "random" results change?
If it's just that some are not as relevant as you would like, you should increase the "match threshold" to make YARPP more strict. Go to YARPP settings, go to the screen options tab to show the "relatedness" options, then increase the match threshold and save.
Finally, the img that you see in your source is there because you opted in to the optional data collection of YARPP settings and usage. This is explained in the YARPP settings page, if you go to the help tab and click on "optional data collection." You can of course turn this off at any time, but I appreciate your participation.