I think that the transliteration system has undergone changes in recent years. Chinese words are spelled "more phonetically" now than they used to be. As an example of the old system, I offer you my friend's friend's name, which was apparently spelled "Qong" but said "Gong". But the word "Tao" made its way into English usage a long time ago, compared to the relatively recent spelling changes, so it stays "Tao" pronounced "Dao", I guess.