What might be mistaken for "strange" about the "the master" is that he "is a person of an excellent disposition" and is " a noble fellow."
What makes the master so noble and excellent, according to Walton, is that the master sacrifices his fortune and the woman he loves for the woman he loves. I know that may seem confusing, but if you have read "Letter II," Walton tells us that because the master amasses "a considerable sum in prize-money" he is able to obtain the consent of a Russian father to marry his daughter; the marriage is an arranged marriage, not one in which both parties are in love; the master is in love with the Russian's daughter, but she is not in love with him. This type of marriage was common during this era.
But once the master learns that the daughter is madly in love with a man of lesser fortune, he forsakes the marriage,and he gives his fortune to this other man so he can marry the Russian's daughter, so you see, what the master does is indeed "strange" only in that not too many men would have done what he does.