Why do all the rings of MH370's transmission locations have the same origin point?
Because the Inmarsat 3 series satellites that were used to transfer the ACARS system data are geostationary. They orbit at a speed and altitude that causes them to stay in the same location with reference to the earth. The signals were received specifically by the Inmarsat 3 F1 satellite which was, at the time, geostationary over the Indian Ocean.