Hi David,
It is very unlikely that they are matching due to the navigation, as that is not part of the post contents field, which is the main source the plugin uses.
I looked at the page source on this post as an example. There is indeed no visual for the keyword “board” but there is within the source code of the post body. Some arguments are printed for the gallery images, which include a parameter keyword “keyboard”, which is matched.
The best possible way to get around this in my opinion is by switching to the index table engine.