Ain't'cha Bitch was exclusive to the Explicit version, Mirror, Mirror was exclusive to the Clean version. Rob said it was done this way because they couldn't clean up Ain't'cha Bitch for the clean release and thus pulled Mirror, Mirror to give the clean fans an Exclusive to make up for it.
The only real difference is that it's not a direct swap of the tracks. Mirror, Mirror appears earlier in the album on the Clean version then the spot left open by Ain't'cha Bitch, thus the track numbers differ slightly.