...Is it mr)smith or mrs)mith? He/she/it???
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The common denominator is anonymity. Mr. Smith is the ultimate name for anonymity. Therefore Mr. Smith denotes the male name, whether or not the person is actually a male.
The other factor is that the Bagster told us he devised the switcheroo from Mr. to Mrs. leaving the Mith as the new last name. He did it to annoy the troll.