I created a cube and then 'ctrl + R' to create a line of symmetry for the cube (Vertical). Then I 'x' to delete the left vertical and add a modifier of mirror so that the cube could be adjust by only changing one side. But when i try to adjust the size by 's', the line of symmetry will move with other line as well, which I did not expect to happen. As shown:
Have I done something wrong?