Few things I noticed on mine. If I ease on the boost it hits 12psi and stays there. If I boot it, its smooth (but real fast) to 15psi, then drops back to 12psi and stays there, change gears its 12psi again. So I think its just a cheap non water filled gauge.
With the boost I had mine on 14psi in winter (with a bit of a cold air set up) I thought it was a tad unsafe because at the time I didnt have a adj fuel pressure reg, so I put it back down to 13psi and found it went faster. I have since got a fpr, and lowered the boost, I find I now get REALLY good fuel economy, where as 13psi I didnt. Strange but hey.
BTW Im also running the stock cooler.