Final answer:
Macbeth is bewildered as Ross addresses him as the Thane of Cawdor because the current thane is still alive. The title was granted to him by King Duncan after the previous thane was sentenced to death for treason, making Macbeth simultaneously wary and intrigued by the witches' prophecy that he will be king.
Step-by-step explanation:
Macbeth is questioning Ross because he has just been hailed as the Thane of Cawdor, a title that, to his knowledge, is still held by another man who is very much alive. Despite being recently victorious in battle and receiving commendations from King Duncan, Macbeth is perplexed by this new honor. His confusion stems from the fact that earlier, three witches prophesied that he would become the Thane of Cawdor and eventually the king.
Despite his skepticism, the first part of their prediction came true when Ross delivered the message that King Duncan has granted him the title after the previous Thane of Cawdor was found to be a traitor and sentenced to death. Through Ross's message, Macbeth realizes the witches' prophecies might hold weight, prompting a mixture of ambition and fear regarding the possibility of ascending to the throne.