I am just speculating that Legendary bought the ammo from a private seller, not from a distributor. Maybe they got it at an estate sale or in a trade. Maybe they got 20 boxes and checked 3; probably they did not know that one box was short. That seems to me to be the most likely explanation; I doubt that they would try to screw a customer out of five rounds of .32. That's not a smart way to steal.
Possibly the seller (or the seller's widow) did not remember or know that one box was 5 rounds short. It is my habit -- for my benefit, not for the benefit of a subsequent buyer -- to identify partial boxes. If the box is more than half full, I leave one of the side flaps on the end untucked. If it it less than half full, I leave both side flaps untucked. That way I know if I am grabbing a partial box when I go to the range.