While I'm not 100% positive, I've always assumed that they commented on the whole movie, and then all those were edited together. It seems like it would be more work to pick out scenes for each one to comment on. And, you're often switching back and forth between participants in the same scene...
*cough* RateThatCommentary.com *cough* :D No, seriously, you should probably check out the highest and lowest ranking ones on there if you want some recommendations on which tracks are good and which are not bringing the goodness so much... The last one I watched that I really liked a lot was...