Unless you have done a deep dive into this and examined hundreds, if not thousands, of kids and tracked them, I am thinking you are seeing a few and expanding it.
On the girls side often once puberty is full on girls slow. They gain body fat, hips widen (in most cases) so they slow. They then need to learn about their new bodies. So will continue to improve even after a little regression. (Here coaches, especially male coaches, need to be SUPER careful about not focusing on weight here).