This is because when you convert the map's ground texture to a Pathfinding file, the file's size has to be cut in half. So a 2x2 square becomes a 1x1 square (1 pixel). And when a Pathfinding file is converted to a map's ground texture, a pixel becomes a 2x2 square.

It is VERY IMPORTANT to keep this in mind because if on 2x2 square is half white and half black, you won't know if (after converting the textures to a Pathfinding file) the pixel will come out completely black or completely white.