Hi Tom,
In your picture, the assembly is upside-down, so really the "bottom" of the seal is sticking out a little more than the "top". Probably because the rubber has taken a permanent set. There is more force on the top when the breech is closed because of the increased distance from the pivot pin (think of it as a fulcrum), so it's my guess that this just came from age.
Try the "tissue paper test". If it fails that, replace it or shim it.
Mike