Don't panic.
not all "500psi" pumps and pump configurations are created equal.
If you have sufficent pressure and flow to rinse quickly, then ignore the gauge for now, and continue to march.
We have a pump done to
GreenGlides specs for out high perf. porty, it'll show 500psi static if not more, but once you hook up a 4 jet Greenhorn it drops to 275-300psi Dynamic pressure, but let me tell you that is an impressive display coming out of those jets.
It's very likely you have a smaller motor and smaller output cam on your "500psi" configuration, plumbing also plays a roll.