If two separated speakers play the same sound at exactly the same level(as heard at the listening position)the sound should appear to come from directly between them for someone sitting directly between them.
The volume is only one factor - a big one - but not the sole factor. The reflections play a large role as well. If the first order reflection is close in time, you end up with a virtual driver (sound source) to right of the main speakers. So instead of the image centered between 2, it is centered across the three speakers, with the third (virtual) speaker being played at a lower level and not in time phase with the other two.
From Adrian's description of his room, I think this is the problem. His boundaries are not equal, and that will pull the image around and force it to move as frequencies change. I lived through this in my last room. I think this article had some useful info on same. If not in this article, check his other articles.
Hodas
