remember the early fuel pressure regulator varies the fuel pressure directly related to the intake manifold air pressure.
because of this it negates the difference in pressure between the fuel side and the injector air/tip side.
On the later dead head steady pressure systems you end up with the injectors flowing more at high manifold vacuum then at high boost.
because at 7 PSI of boost... fuel pressure is effectively 50-51 PSI whereas at idle fuel pressure is effectively 64+