Tldr; earth is not a perfect sphere. Rotation of the earth makes the middle fatter. Density of areas of the earth make gravity change. The lowest density area is off the coast of India and it confused people since 1948. We recently did a computer Sim of 19 scenarios across 150m years of tectonic plate movement and magma density and temp. A couple scenarios predicted "plumes of magma" with different density under that area. The models didn't accurately predict some other known tectonic and volcanic activity, so really we just have a really good preliminary theory.