You really don't know how a turbo works, do you.. Actually, you don't really know how a gauge works either..

This has nothing to do with the gauge; the gauge just reads pressure, it doesn't care at all about RPM. Digital pressure gauges work exactly the same way, all they do is display pressure. If you don't have 7 psi of manifold pressure, the gauge doesn't display 7 psi of manifold pressure.