I have experienced the sensation, watched the instruments and thought about it. My conclusion is as others above: Most of us unconsciously move to the outside of the lane when passing, but autopilot does not (yet). It usually holds the dead-centre of the lane, and hence is noticeably closer to passed or passing traffic than we are all used to. This is certainly something they could adjust, since the ultrasonics can measure the distance and bias the lane-holding position a little.
This is particularly noticeable on sweeping curves. Again, most of us tend to hug the inside of the curve, either properly at the apex, or improperly throughout the curve. This results in being further from the adjacent lane on right-hand turns (for drive-on-the-right countries). Again, AutoPilot does not (yet) do this, and so it can feel like you are unnerving close to the other lane, but if you watch the display on the IC, the car thinks it is well-centred in the lane.