I think in this case it's a case of not being able to easily determine if "mov ebx, 0" is copying a value into a subregister, making the result dependent on pre

I think in this case it's a case of not being able to easily determine if "mov ebx, 0" is copying a value into a subregister, making the result dependent on previous operations, or if it's clearing a register. With xor (and sub too, I believe) the processor assumes that no such dependencies exist and can execute the instruction in parallel with others.