It would be easier to understand if you were a bit more specific in the description of the orientation (i.e. driver's side or passenger's side instead of right or left). If the passenger's side is slightly lower, it's likely as erhayes said, offset to compensate for the driver's weight when they are actually in the car. If the driver's side is lower, it's quite possible that the LCA bushings have simply worn down over time. A driver will always be present for every mile driven. The same can't always be said for the passenger, which means that the driver's side bushings will wear down faster than the passenger's side, creating the difference in ride height. It could also be due to the battery or other heavy components on the "sagging" side.