Beats Per Minute

Usually when I run, I listen to really upbeat punk, pop, or rock music (i.e. The Distillers, Kylie Minogue, Courtney Love, etc.). But lately, I've been listening to dark, moody tunes (PJ Harvey, Elliott Smith, Fiona Apple, etc.). And I seem to be having more productive work-outs. I don't really understand why. It might have something to do with trying to run with the beat of the music, and since the slow music has almost no beat at all, I run as my abilities allow. Hmmm, interesting.