It has to do somewhat with the longitudinal tree shape. If you flip the saddle over, the curve between the panels should match the horses back shape. You can try something like a bates that tends to fit horses with a flat back shape.
My saddle fits my horse well except in this aspect, but the other way, the tree wants the withers to end further back than they do, so the saddle shifts forward. For me its gotten better as he has muscled up, but the best thing I've found is the ecogold secure pads. They are pricey but absorb most little discrepancies between the saddle and the back because of memory foam.
You should also try a kn saddle if you can get your hands on one..
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