He mentioned that in the article: > What's interesting is that while the most representative songs for each band are mostly their own songs, occasionally other bands songs creep in. For example, "Wrathchild", is an Iron Maiden song, not a Diamondhead song.

I don't think it's a problem with the song data. When the algorithm picks a song it considers most lyrically representative of a band, it chooses from all the songs in the dataset, and it doesn't always pick a song by that band.