I've noticed someone in front riding while rear wheel slightly inclined either right or left, completely not understanding the reason.
I think that could lead to shoulders pain due to poor posture in long trips.
Everyone's body is slightly different from side to side with one side being heavier, fatter, longer, more bony, or just more naturally bent. So when a rider gets on a bike, he usually just sits where it's the most comfortable, not necessarily exactly in the center. The bike will lean slightly one way or the other to compensate. In addition, the bike could have more gear in the bags on one side or even a change in side to side weight from the change from stock exhaust to newer two-side pipes.