Look at the drawing from the Bentley manual. It shows the bushing sleeve pivoting on the bolt and the shoulder. They have to be the same size OD. Put a mic on them; I think your photo is misleading because of the angle the picture is taken with.
What I think happened to bugger the shoulder up is with having only 1 washer in there, the trailing arm pulled to the outside of the car, then fell down, off the shoulder and onto the threads of the bolt. It rubbed back and forth for who knows how long, therefore wearing the parts together. Gordon, what do you think of my theory?
Once you have the 2 washers installed, it shouldn't happen again. BTW, the reason for 2 washers is it spreads the rotational friction over a greater area.