To back up what SL1 said, the well-aged sectioned case below shows a clear donut. while the once-fired one does not. The old case also has inside reamer marks from when the previous donut was removed. In long neck cases like the .30-06, it's most common that bullets don't seat deeply enough to get squeezed by the donut. In shorter necks, the possibility exists. Presumably seating pushes it outward so the taper of the neck is slightly expanded.
The donut forms during full length sizing. The die first narrows the entering case, which elongates it, then, after the shoulder of the case hits the shoulder in the die and starts to push it back, The shoulder brass slips upward and inward and fails to go completely into the neck, with some pushed inward at the corner with the neck, forming the donut.