Besides the comments from Alk8944, looking at the pictures gives us a clue.

The head of the case is solid, the case walls taper as they go up. The ring forms at a point where the pressures are enough to expand the brass, but the brass is thick enough not to spring back to the same extent as the thinner case walls. As noted, this is normal.

Sizing dies generally don't resize completely that far down the case walls, they do go far enough down to resize the case to dimensions that allow reloading and normal operation.

They do make small base sizing dies for some rifle cartridges to return that area of the case closer to original dimensions. No one has demonstrated a need for that with pistol cartridges.