To answer your question directly, there isn't a photographic reason why it would be shifted from the optical axis, other than packaging, electronics, weight placement, or other industrial design compromises that are specific to that camera or model line.

But to fix the issue so that the optical axis intersects the axis of panoramic rotation, the easiest solution is to buy a camera-specific adapter plate to mount your camera into a tripod clamp mount. For instance, the Acratech quick release plate for Canon G11:

Acratech quick-release plate for G11, from Acratech.net

The intention of these plates to is stay mounted on the camera long-term. Note that as opposed to generic QR plates, the screw for the plate is very close to the right edge. That's because the G11's battery door hinge is close to the tripod mount socket on the bottom of the camera. This specific plate allows you to remove the battery without removing the plate; few generic QR plates, if any, will be as accommodating. Once mounted to your G11, this plate is centered under the lens.

Of course, the plate is useless without a corresponding QR clamp on your tripod. Search your favorite online retailer for "Arca Swiss clamp" — they can be found for as little as 10 USD.