Not sure what you mean by "distortion", but...
If you were using only 2 surround speakers, placed along your sides and slightly behind you (±110-120°), some sounds would image at your sides while other sounds would phantom image in between (appear to be coming from behind you).
Of course, you have to be sitting in the sweet spot to hear this directionality. If you're sitting left or right of centre, then the sounds you were hearing behind you will collapse to the nearest surround speaker.
When you play back a 5.1 soundtrack on a 7.1 speaker layout using PLIIx, the sounds that were going to image behind you are extracted from the surround channels and sent to the speakers behind you. The leftover information remains in the speakers at your sides.
Now you can be sitting anywhere and sounds that were intended to come from behind you will always come from that direction. No magic involved, just an additional pair of surrounds behind you (makes it difficult for those sounds to come from any other direction).
Same directionality with greater imaging stability.