The intensity of sunlight is relatively high in the Equatorial regions and relatively weak in the polar regions. The steady-state concentration of stratospheric ozone is inversely dependent on the sunlight intensity, since the stronger the sunlight, the higher the destruction rate of the ozone by absorption of solar UV. Thus, stratospheric ozone concentration is low over the Equator (in this map, it's about 250 DU) and high over the poles (in this map, it's about 350-400 DU over the North Pole...we cannot tell what it is over the South Pole, since there was an insufficient amount of sunlight there for measuring the ozone concentration at the time of the year the observation was taken).