Other possibilities: left and right back wall or side walls are asymmetrical, eg. right side has more open space; your preamp or amps output power is not equal/balanced. You can use left/right balance know of your preamp to adjust. In my case, my two mono class A amps has tiny output difference (even they are within spec of error tolerance).
I have a question that bothers me, on the explanation sound stage and imaging: On one guitar playing, why do I hear the bass string on the right side and the lead strings played on the left side? There are times other musical instruments happened to be in the same situation too, the musical instrument is split into two pcs, one on the left, one on the right.