When one part of a leaf/twig/whatever is damaged before it stops enlarging, that edge or part dies and stops growing while the rest of it continues to grow to full size. It's like pinching a balloon before you blow it up. The part you are holding can't move with the part that is expanding with air filling it.
That's different from contorted species like Flying Dragon or Harry Lauder's Walking Stick (contorted Filbert) where one strip of the stem is growing at a different, slower rate than the rest of the stem which is a genetic fault, but causing the same outcome.