As a wild guess, you probably have other (hidden?) images inside your music folders that MusicBee is picking up.
So inspect the folders for these specific tracks to see if that's the case or not.
And as a 3rd confirmation, you can open the tag editor and go to the artwork tab to see if there are indeed no embedded images.
Edit:
Since I only work with embedded pictures for my music, I am not really familiar with how MB treats tracks which fetch album covers from folders. Does it consider those tracks to be without/missing artwork? If that's the case, then if you have MB set to auto-retrieve missing artwork for the playing track (preferences > tags 2), then it's possible that that's where the other album covers are coming from.
I already spend hours on end on social media. Might as well spare a few of those to a greater purpose here.