Chris, AFC single point only uses one focus point, the one you select and does not "pick" another focus point at its choosing.
The camera will pick another focus point if the Area Mode is set to Dynamic (9pt. 21pt. 39pt.) or 3d tracking. Using AFC with single point AF you know exactly what point the camera is using. Your movement will cause the camera to adjust focus which is fine as long as you continue to keep that single selected point on the area you want in focus. You are confusing the autofocus mode (AFS, AFC, AFA) with the Autofocus AREA Mode (Single pt, dynamic pt, 3d tracking).
Marc There are always two people in every picture: the photographer and the viewer.-Ansel Adams