On his discussion of the term homoousios, Barth noted how the fathers at the Council of Constantinople (particularly the Cappadocians) rendered it to mean “the equality of essence” between the Son and the Father while the West, which followed Athanasius (an Eastern bishop), rendered it to mean “the unity of essence” between the Son and the Father. Thus, “[p]olytheism thus became a constant threat to Eastern theology. On the other hand, it must be admitted that the idea of oneness of essence made Modalism a constant threat to Western theology.” (I/1, 439)
The difference is influenced by their different historical contexts. The problem of Athanasius was Arius who argued for the ontological difference between the Son and the Father. And this brouhaha led to the Council of Nicaea in 325. On the other hand, the Cappadocians were concerned with “the distinction and autonomy of the persons, which had received little emphasis in Athanasius” (I/1, 439). And the matter was settled in the Council of Constantinople in 381.
Hence Athanasius was asking how the Son and the Father are really One while the Cappadocians were asking how the Son and the Father are really Two. To be truly trinitarian, then, the East needs the West and the West needs the East.