There is a purpose.....
At maximum pinch there is also a maximum compression. When the engine is broken in and the pinch is (almost) gone the piston is actually floating on an oil layer in the sleeve. The huge pressure of the combustion can push some along the side of the piston so there is a tiny compression loss. That can be compensated by removing one shim.

But it is all about how the engine is configured at the start, some engines have that extra shimm from the start, some do add an extra shim in the box, some do advise to use an extra shim and others do not anything.