So, in your case it acts the other way around, that's indeed not exactly what someone would think ☺
It looks like it has got something to do with the free floating hammer of the FX guns.
I've written it as advice in many other threads as well, as people tend to max out the HST on their FX to get the highest possible velocity. We'll, to be short, it isn't ☺.
There is a way of diminishing returns here, when you start to increase the HST on any FX, eventually the speed will drop instead of rise, while the air consumption gets horrible.
In your case, it seems that the rubber ball is preventing the valve to overshoot it's optimal setting, hence the increase in speed.
Just as a sidenote, leaving the HST on max. and turning in the valve control too much doesn't really sound good for the rifle. The hammer is smacking the valve at full force, while the valve can barely open because the valve control knob is blocking the valve stroke. It seems better to also decrease the HST setting slightly when turning in the valve control knob in pretty far.
With this specific FX phenomenon in mind, you can also try out if your speed increases with the valve control all the way out, and your HST set at a setting lower than MAX
regards,
Gijs