The thing is that the very thick and voluminous sleeve seam allowance can either be stuffed inside the sleeve, which will make the sleeve head kick straight up, and do a really dramatic and big sleeve head... But also would need a bunch of sleeve stuffing and support to make it sort of round off into the arm.
On the other hand, if you lay it against the bodice instead, the seam allowance won't fight you, and the sleeve will sort of... lay flatter and softer, and not need any more support than the sleeve lining and arm inside it will give. This is how I generally like them, too.