Why is that?
If we compare girthing a horse to tying a leather belt around our own chest tightly, where would you put it for the least discomfort? I bet you would put it right under your arm pits, or just an inch below, for maximum comfort. If you put it further down, you restrict your breathing and start to bruise or even crack your ribs. The same goes for a horse, though there is a little more room for error. If the horse has a little condition, the girth will slide naturally into the so- called girth channel. If the saddle is designd to sit just behind the shoulder, and the rigging doesn't fall to meet the girth groove, the straps will likely be pulled forward at an angle to sit there, or the horse will be uncomfortable and the saddle unstable if the girth sits too far back. If the horse is very fit and has no belly, the girth will slide further back, but then it is in danger of also sliding too far back. That is where the breast plate comes in. Where a good tree should always stay in the right position, adding weight of rider and movements of the horse, by the laws of physics a rigid tree will move, if the girth is not holding it in the right position.
Just my theory on all this