In the short story, "The Open Window," why does Mrs. Sappleton call Mr. Nuttel 'a most extraordinary man'?

When Mrs. Sappleton calls Mr. Nuttel  “A most extraordinary man”, she does not mean it as a compliment.  In fact, she hardly knows the man. 

Mr. Nuttel has been told by his doctor that he needs some rest and relaxation, so his sister convinces him to spend some time at a rectory in the country. His sister knows that he will become a hermit in the country if he doesn’t know anyone, so she writes him a letter of introduction to people she has met, but with whom he is not acquainted.  

Mrs. Sappleton is not ready to receive visitors yet, so her niece, Vera, entertains Mr. Nuttel until her aunt arrives.  While waiting for the aunt, Vera tells Mr. Nuttel about a tragedy that happened three years before on that very day. Mrs. Sappleton’s husband and two brothers had gone out hunting with their faithful dog and never returned.  They would come in from a hunting expedition through the window, and so Mrs. Sappleton left the window open in hopes that they would return that day.

When Mrs. Sappleton arrives, she tells Mr. Nuttel that she is waiting for her husband and brothers to arrive and hopes that the open window doesn’t bother him.   Mr. Nuttel is very uncomfortable with this topic, so he tries to change the subject. 

“The doctors agree in ordering me complete rest, an absence of mental excitement, and avoidance of anything in the nature of violent physical exercise” (Saki 2).

He continues to elaborate on his physical maladies, but Mrs. Sappleton is half-listening to his ailments because her eyes keep going to the window.  Finally, just as the sun is beginning to set, Mr. Nuttel sees three figures walking across the lawn carrying guns over their shoulders.  Mr. Nuttel grabs his coat, hat, and walking stick and runs out of the house.

 Mrs. Sappleton doesn’t understand.  By “extraordinary” she means “strange.”  She tells her husband, who is very much alive, Mr. Nuttel

“…could only talk about his illnesses and dashed off without a word of goodbye or apology when you arrived.  One would think he had seen a ghost.” (Saki 3).

No one knows that Vera told Mr. Nuttel a fantastic tale, and Mr. Nuttel felt he had seen a ghost.

My copy of the story is from the internet so the pages may not coincide with your copy, but they should be close.