Why is the Emperor of the Roman Empire called An Dun, King of the Great Qin Dynasty, in Biography of the Western Regions?

The word "Daqin" is the name of the ancient Roman Empire in China, and antonini was Antoninus Pius (A.D. 166), the emperor of the Roman Empire's antonini Dynasty (five wise emperors).

As for why the Roman emperor was called the king, it was because of China's ancient worldview: one day can't last two days, and there is only one son of heaven and one emperor. It's not bad for a foreign monarch to be king.