I think this is because that speaker has a lot space behind and to the side whilst the right has less and more complicated space behind and a closer side wall.
A reasonable enough sounding assessment of the situation. So my question is, how is balance going to address this problem? It won't.
Balance will make one speaker louder. It will indeed shift the image a bit to whatever side you make louder.
But that's not all it will do. It will also do real damage to the depth and solidity of that image. So what will happen is, you will be real happy at first. Got my center all nice and cetered! Big whoop. Then you notice sounds that used to be nice and 3D in the stage now seem to be coming right from the speaker. Now not so happy. Not happy at all.
You seem to have a pretty good handle on the situation. Just need to work out how to fix it any other way than changing the balance.