Because it isn't just about the timing...just because you increase flow doesn't mean the car was making 2-3* less than stock timing and knocking from the factory. All headers have done on some cars is put them back to factory tip top shape, and then...you're running a stock pulley on a car that makes 5-7psi with manifolds and a poo-poo DP setup...and expect what...3-5psi after the headers? Then you need to pulley down to compensate the loss of boost from the flow to get back to square one...and then wheeee, KR on a smaller pulley.
It's a fun cycle, isn't it?