Ok, there is indeed something very strange happening to the last.fm API. When you look at https://www.last.fm/music/Nine+Inch+Nails it shows as top tags industrial, industrial rock, electronic, rock, alternative, instrumental.
But if you query their API for the top tags of Nine Inch Nail (which the plugin does) it returns as the top 5 “emocore”, “rock en espanol”, “Sweet Dreams”, “Stylish” and “groove metal”. The word “industrial” does not even show up in the entire list. The top tag “emocore” has only 58 uses, which looks pretty low for an artist as popular as NIN. When querying the API for track to tags, let’s say for Hurt, I don’t even get any tag.
But this is an issue with last.fm’s API, it has been reported already at https://getsatisfaction.com/lastfm/topics/gettoptags-getting-wrong-tags