I can't test it in DS right now, but I'm inclined to think it's a Poser thing based on the behaviour of the mirror function with the morphing tool (even in a perfectly symmetrical figure, when you mirror x, the mirrored side has varying degrees of distortion. I haven't tested the y or z mirror.)

On a side note, if I'm making custom morphs that require both sides of the face to be morphed but different and I want to do a reversed version, is there a way to swap the whole shebang at once? For example, a bite lip morph which requires the lips to be distorted on both sides of the mouth at the same time in order to look correct. I'm trying to get around doing it the long way. ;-)