There could be a number of variables at work here, but the most likely explanation is the reinforcing effects that the adjacent surfaces have on the speaker nearest the corner. I'm actually surprised that the difference isn't greater.
The analog audio output of DVD players and CD players should be spec'd to 2 volts, but there can be significant variations between channels sometimes, which may account for such differences.
Try reversing channels on your speaker connections to see if the disparity in output remains with the left speaker. If it does, I'm certain it's boundary effects at work.
Axiom Resident Expert