in addition to Snoppies comments, MP3 is compressed, so even a perfectly centered vocal will give some residual noise. The higher compression the worse you will find it as the 'edges' lose definition. I use 320kbps for all of my MP3's

The Audya analyses and 'layers' the left and right parts in real time, when it sees identical data in both channels it is able to neutralize that element of the sound, hence why you still have some breakthrough with MP3 files.

WAV files are far better as they are uncompressed...the drawback is more disk space.