細い喉で、尖った喉仏（のどぼとけ）の動いているのが見える。その時、その喉から、鴉（からす）の啼くような声が、喘（あえ）ぎ喘ぎ、下人の耳へ伝わって来た。「この髪を抜いてな、この髪を抜いてな、鬘（かずら）にしようと思うたのじゃ。」
下人は、老婆の答が存外、平凡なのに失望した。そうして失望すると同時に、また前の憎悪が、冷やかな侮蔑（ぶべつ）と一しょに、心の中へはいって来た。
My default interpretation here would be, since 平凡 here is a na-adjective, that it would mean "despite". That is, the servant was disappointed despite the banality of her answer (that went against his expectation). The English translation, however, goes:
The servant was disappointed by the unexpected banality of the old woman answer.
That is, the counter-factuality of "despite" is missing. Only causation is left. Is it due to the translator's freedom of interpretation or have I misunderstood this grammar point?
How is のに used here?
