And by "explain", he means that he's guessing that they "simply ran out of 36-inch guns" and decided to substitute 40-inch guns.
It couldn't be more obvious that Klein's
was definitely running out of the 36-inch rifles in the early months of 1963, because they completely stopped selling (and stopped advertising) the 36-inch variant after February of '63. Every Klein's ad after February shows the "40-inch Carbine".
It's not hard at all to figure out what happened with Oswald's order in March (except if you're a conspiracy theorist, of course).