Could just be room acoustics and moving speaker few inches closer than right.
Could be your hearing also at times ...I have tinnitus and some days my hearing is not as clear as others ..With my Raven tube Reflection I have been thinking of putting a balance control on it for that reason on days I just feel off..
But I find I am moving more to left side to make up for the lack of control of balance ..My living room will not allow me to keep the speaker 3 inches I need out and moving 80 lbs each time is not best condition..
And to be honest it could be just the way it was recorded in some songs.. I have found some recordings are not total balanced and hear another song and the left sounds louder than right...
Could be few things going on ..