because volcanoes in the center of the ridge push the pre-existing seafloor on either side of it away from it at equal speeds
also when the earth's polarity flips, the crystals on either side of the spreading center are affected equally, so the bands are always the same distance from the ocean ridge
also because when the magma solidifies (which is what "freezes in" the magnetic orientation), the magnetic field of the earth exerts the same force on both sides of the ridge (the midline of the spreading).
