The "clumping" the author refers to is within genres. In a perfect random number generator any of the possible output values is as likely to appear next in the

The "clumping" the author refers to is within genres. In a perfect random number generator any of the possible output values is as likely to appear next in the sequence as any other. PRNGs are not perfect, but there is nothing either unfair or non-uniform about two blues tunes showing up next to each other, if they happen to be stored in flat list of songs and indexed by ordinal. I think you need to reread the piece.