why is oxidation state of 2 sulphur atoms in thiosulphate ion +6 and -2 and not +4 and 0?

According to rule assigning oxidation numbers suggest that each sulphur atom in thiosulphate ion S₂O₃^2--has an oxidation number of +2.However ,the structure of this ion suggest that 2 sulphur atoms are not equivalent and are quite different.One sulphur atom is at the centre of the tetrahedron bonded to four atoms (1 sulphur and 3 oxygen atoms) which is similar to S atom in sulphate ion(SO₄^2--).Therefore an oxidation number of +6 is assigned to central S atom in thiosulphate ion like the S atom in sulphate ion,and an oxidation number of --2 to each surrounding atoms including the second S atom.Thus we can say that two S atoms in thiosulphate ions have oxidation numbers +6 and --2 respectively.