Actually, I think it's more up in front than it appears. If you look at this photo which may be the same saddle:
I am guessing with such a straight dressage type flap, this saddle is supposed to have the flap fairly straight up and down rather than forward toward the horse's shoulder. Which means the saddle is too narrow, which is pushing the rider back and off balance.
That saddle as with many older stubbens appears to be meant to "swoop" up in the back of the panels rather than having them sit flat on the back.
I am also basing this on the fact that if the rider sits on the seat where it is lowest when it is on the horse, her leg hanging down in a dressage position would be on the back half of the flap.
Which is probably why she is riding with such a short stirrup...so her leg remains on the flap.
I agree with tinyliny that the stirrup bar placement also is critical. But in this case I believe the stirrup bar might be correct if the saddle were to fit the horse.
This is a case where no amount of proper position will fix the problem. I've seen excellent riders sit on a saddle that is off balance like this, and they instinctively sit where they need to even if they're half off the back of the saddle! Which shows that the saddle does not fit the horse.
It is difficult to see that the saddle might not fit if you assume those back panels are meant to sit flat on the horse's back. But quite a few dressage saddles are designed to have the cantle go higher than the pommel, so just putting those two level does not always mean the saddle is level.