My understanding is that when the signage system was designed, it was determined that the actual town borders did not always provide the best spot for the welcome signs. Issues they considered included property ownership, ability to dig footings at given locations, drainage, "line of sight" for motorists, attractiveness of the surrounding area and a host of other factors. In a perfect world, the welcome signs would be at the borders. The green county signs delineate borders. The way finding signage system as a different aim.