The horse is not 'how overflexed,' it is just a few degrees back of the vertical. This can happen for as trivial a reason as the rider simply half halted. The position of the horse is generally very good, at that moment, it got a little behind the vertical. That would have lost the horse some points in competition. In schooling the position is not going to be perfect every second because the rider is working on various things, for example she tries for a little more engagement or a little more on the spot, the horse loses its balance a little, the rider goes forward, then try again, the world won't end. Good grief I have seen so many Western ridden horses with their chin on their chest and no one bats an eyelash. The horse is supposed to have its neck lifted up and its head on or near the vertical when piaffing. And in one picture it is slightly behind the vertical. Most of the time the horse's position is very good.