According to Federal Highway Admin rules the clearance measurement should be at the lowest point on the bridge portal. In the photo that would be where the angled brace from the top chord joins the side truss.
If you stay dead center, you gain about two and a half more feet. This is the same as the tunnels on the Skyline Parkway. Posted at 11'-6" but actually 14'-2" at the center of the road.
That said; remember that the measurements may well have been taken by a Summer Intern working for minimum wage and not really interested in accuracy.
