So far as I know, there are more in the Northern hemisphere because the Earth is tilted with respect to the plane of the Milky Way and more of the Milky Way is present above the Celestial Equator than below it. Even the distribution of nearby galaxies is skewed in this way. Just the luck of the draw I guess. I would have been astounded if the numbers were exactly equal especially since the Sun does not lie exactly in the central plane of the Milky Way, but is about 50 parsecs or so to one side.