Possibly the same way Fallout Boy and System of a down can be grouped. There is a phenomenon in music ‘similarity’ systems (remember Pandora?) that, because the process of actually analyzing music and classifying it is work, tries to offload the work to elsewhere, and often what really happens is things get grouped not by qualitative similarities like mood, rhythmic complexity, tone etc. but by the quantitative and easy ‘these two were liked by the same group of people, aged X~Y, so they must be similar.’