Plus the Earth is erratic in its movements compared to atomic clocks. When they switched the definition from a fraction of an astronomical day to wavelengths of

Plus the Earth is erratic in its movements compared to atomic clocks. When they switched the definition from a fraction of an astronomical day to wavelengths of a particular Cesium photon, they would have to add a leap second a few times a decade. Last year they were considering subtracting a second because the Earth speed up its orbit for unknown causes.