For some reason the photo graph won't load for me.
While I can't offer any specifics without looking at the graph, I can add some general knowledge:
PME is governed by all aspects of mechanical resistance, including those not often discussed. For example, a stickier lube/grease in your cylinder creates more friction between your piston o-ring and the cylinder, lowering the PME threshold. Heavier BBs will do this also, as they create more mechanical resistance. This is because a heavier a BB requires more energy to accelerate, hence why air volume ratios are very important. The more mechanical energy the BB requires to move forward, the more pressurized air builds up behind it before it breaks over the hop nub, the more resistance is applied to the piston's forward path.
The curve is actually quite steep--a build could have no PME on .20g but suffer mild PME using .25g and heavy PME with .30g-.32g. This is far more prevalent in DSGs, where the cycle is shorter, the timing is stricter, and the piston is moving faster (resistance increase on an exponential curve relative to velocity).
Hop packings also affect PME threshold. An off-spec hop packing can be slightly too tight, causing more resistance and lower the threshold. Off-spec BBs do the same thing.
You can raise the PME threshold by using a cylinder with a smaller effective volume (porting farther forwards). This allows the piston more time to gain momentum before experiencing air mechanical resistance, effectively increasing piston speed, which increases air pressure, increasing FPS in some cases and increasing efficiency/consistency.
A wider inner barrel and wider nozzle will also raise the PME threshold.
The same applies for the drivetrain side of things--a more efficient electrical setup with less resistance leads to a faster rate of fire which leads to a lower PME threshold for a certain setup, irrelevant to gear ratio, spring weight, etc. So, ironically, a better setup here leads to a lower PME threshold. This goes for how thick your grease is, how well the gears are shimmed (truly bad shimming where extra resistance is caused, not just noisy shimming), and other factors like gear meshing.
Contrary to popular belief, polishing your gearbox around the gears to reduce resistance does nothing. It's actually really silly, since the gears should never contact anything but themselves, the piston teeth, and the bushings/bearings.
