It may be that the OP's gps was made for use in China, which automatically applies a counter-offset to compensate for the offsets in the official maps. I don't believe maps of Hong Kong and Macau have the offset, because the laws requiring an offset do not apply in neither Hong Kong nor Macau. If the OP's gps applies a counter-offset to a non-offset map, it will create an offset in the opposite direction. Sorry, I don't know how to fix this problem.