So I'm having this same issue too with Illustrator CS6 on Mac. It seems if I set the left most object flush to the edge of the art board then it doesn't distribute properly. I've tried aligning to objects and aligning to artboard as well as turning off and on the option for use preview bounds and what seems to be happening is that left object on the edge of the art board stays in place and the right most objects move. Toggling options on and off I was noticing that my right most object just kept inching more and more to the left as it was adjusting the space.
Now if I don't use the edge of the artboard for the left object it works just fine and my spacing distributes evenly. Very frustrating for setting up my files.
Has anyone figured out any solution to this issue?