GCJ-02 strikes again! Your military uses inaccurate maps and a secret algorithm for determining what the error in the map is at any given position as a means of slowing down foreign invaders! :-)
Although... I say it tongue-in-cheek, but depending what sources the team used to determine the placement of shore batteries for Chin China, it might not actually be a joke, that's exactly what modern-day, real-world China does. But, on the other hand, the mismatch might be from the use of coordinates from records from approximately the timeframe of the game, in conjunction with a modern, more accurate, map.