Well, I think the angle of the tree is slightly narrower than a QH tree, they have more 'rock' in them, meaning they are more bowl shaped and if set on a flat stand they will rock back and forth like a rocking horse. The tree and skirting is also shorter, with a rounded skirt in the back to give more relief at the hip for Arabs who typically have a shorter back. The slightly shorter skirting and the 'rock' is good for shorter backed horses and those with a large wither. It keeps the saddle from bridging them.
Ohhhh, ok. That makes sense.