Sorry, that's wrong.
One way fill valves have a tiny spring inside, just enough to seat the valve, as you pump the pressure in the hose exceeds the pressure in the gun so the valve is forced open, the two pressures then equalise & the spring seats the valve closed again.
Next stroke the same thing happens all over again.
The gauge on the pump shows the same pressure because hose & gun equalise, because all pressure between open vessels will equalise, which then allows the tiny spring to work.