There is normally a slight bulge (actually up to about .006") at the web (about 3/8" above the base). This is where the brass of the base thins into the case body. This bulging leaves the base smaller in diameter than it is at the bulge.

This less diameter of the base itself might appear indented, I suppose, to someone not sure exactly what they are looking at.

Does loaded ammunition freely chamber and extract without being fired?

Does ammunition loaded in fired brass freely chamber and extract?

Does just-fired brass freely chamber and extract?