The flat bit is mostly the washer as bought, the really swelled bit is the wettest bit and has swelled just beacause it isn't constarined, a small bit inboard of the fat bit is the bit that actually seals, from swelling up but being constrained it is only a tiny area and the crushed washer is mostly completely dry. Any cycling of that joint face from say dragging yourself out of the bath would have the wet area spread and eventually get a weep, but in general fibre washers do a great job, rubber washers squirm if overtightened, and they can't take out imperfections in the mating surfaces anywhere near as well.