Consider a C function like this:

void f(int, int, int, int, int, int, int, int, char x, short y);

In the standard AArch64 ABI x and y get spilled to the stack as eight byte quantities (as if they were assigned to a register and then spilled) but in the Mac ABI they are spilled to the stack with their original size and natural alignment. AArch64 CallArranger needs to handle this difference.