STEREO SEPARATION and dolby pro logic encoding. The higher bitrates have a greater degree of stereo separation and retain more dolby pro-logic encoding. This goes to personal preference. Do you want the sound to be more ambient or do you want similar sounds to seem like they are more centered? So, with less stereo separation, a lower bitrate can have slightly better vocals of the lead singer because more of those vocals are produced in mono at both the left and right speaker.