Tuning n00b question, how can higher tank pressure give lower velocity over a chrono?

So extremely new to PCP, zero background.

In watching and reading many things, I see people run shot strings that are hard to make sense of.

Specifically a recent example was a Benjamin Marauder Pistol.

The tank at 3000 PSI fill was shooting lower velocity until the tank was down to 2500 PSI, where it settled in at a good 40 FPS higher for a run of about 20 shots with a deviation of about 6 FPS until it dropped off again at about 1800 PSI or so.

What is the tuning aspect that is causing lower velocity over a chrono at an actual higher tank fill pressure?

I do know this pistol is not regulated and I'm aware of that but what are the physics around it having lower output velocity until the tank fill pressure actually drops?

Is it hammer pressure that does this? Why does this happen?

Trying to make sense of the mechanics and their effects to understand how tuning parameters interact.
 
Yeah thanks, I was just going to ask if that was the case. So the hammer has to hit harder to make use of the higher PSI to open the valve.

So this explains that if you want a higher FPE tune, typically you need a higher pressure fill and in turn heavier hammer strike. The length of the strike would also come into play as that would control how long the valve was open I would imagine?