The forcing cone is subject to a LOT of stress on firing, and will slowly erode over time.
Low intensity loads, like .38 Spl., normally take a long time to do this, but high intensity rounds, such as .357 Mag., .44 Mag., or some of the newer ubermags can REALLY play hell with the forcing cone in short order.
I have seen forcing cones, normally on guns with many thousands of rounds through them, that have been pretty badly washed out.