I think the numbers are close because the Milky Way and the Andromeda Galaxy make up almost the entire mass of the group i e there are many other galaxies but they are MUCH smaller The Karachentsev paper where the local group estimate of 1 29 10 12 solar masses comes from mentions 5 8 10 11 and 7 1 10 11 solar masses for the Milky Way and Andromeda respectively Those estimates are somehow derived from the local group mass estimate however instead of a method focused solely on the Milky Way