Vanishing Leprechaun Optical Illusion

This classic puzzle dates from the 1960s, although there are variations of this puzzle that are much older. There are 3 strips of card, one long piece and two shorter pieces.

Put the 3 pieces together and you can see 15 Leprechauns. However swap the two smaller pieces around, and suddenly there are only 14 Leprechauns. How did that happen? Where did that pesky Leprechaun go?

This is a version of a type of puzzle called a 'missing area paradox'. Except that here we are not missing an area, a whole Leprechaun has gone missing!