Okay, that is then just the all stereo option that duplicates the same sound in each side(and both mixed to the center speaker). It isn't surprising that the smoothness of the room response would be improved by having the same sound in multiple speakers, but that generally isn't viewed as the best mode for typical use. It's often said to be well-suited for parties, etc., where a more uniform sound is wanted for many people in a room, but it isn't really natural-sounding to have exactly the same sound as is coming from the front also coming from the side and back speakers.
The difference between that and DPLII(or x), which I use most commonly for any type of two-channel material, is that the DPLII processing detects the phase differences in the reflected sound which has been mixed into the two front channels(there was no place else to put it) although it originally came from directions other than the front at the actual performance. It then extracts the out-of-phase material which represents reflections that came from the sides and back and steers it to the surrounds where it belongs. The result is a little closer to the live listening experience.
