BP - it's a safety valve - it only operates when oil at acertain pressure dislodges the ball from its seat.
I thought so. But in that case, why would anyone want to mess with the setting of that valve once it's been calibrated by the factory? (Except to re-calibrate it to the same value?)
You said "either the prv has been adjusted to improve pressure" but I can't place that, unless you suggest that the prv previously had been set to open at 50-70 psi (which is apparently well within the green arc) and has now been reset to the 90 psi setting (which is apparently at the top of the green arc).
But that would suggest that it's a regulator valve, not a safety valve.