I don't get it as bad as this, only a patch about like 5cm on the ledge which gets covered by the door when it's closed.... I think there's a hole in the bottom of the door as like a if any water some how gets inside the door it leaves via that hole, if there's still anything left from the manufacturing that might have still been in there it's the water like dissolving it and coming through the hole, drying on the ledge and sills......it then gets lesser as the car gets older because the gunk has worked it's way through and there's nothing to make the mark