That's normal. Ignore the specs unless you have a scan tool that can read fuel pressure. The spec is as it is read through the fuel rail pressure sensor. It will NOT match a mechanical gauge. In fact, a mechanical gauge is of very little help on that electronic retunless system. The pump would have to be nearly dead for a mechanical gauge to work. The 2 do not match because the FRP sensor is referenced to manifold vacuum and the mechanical gauge is referenced to atmospheric air. To get the 2 to match, you will need to unplug the vacuum hose to the FRP to get the FRP referenced to atmospheric air like the gauge is.
When using a scan tool to determine a pump problem, you'd not only look at pressure but also percentage of pump "on" time (40psi at idle with 23% duty cylce from PCM is good, but 40psi at idle with 45% duty cycle from PCM is bad). Then you'd also have to use an oscilloscope to check the output of the fuel pump driver module to ensure it is ramping up the pump "on" time per the command from the PCM (just because the PCM is commanding it, doesn't mean the driver module is running the pump right).... the twist there is that whatever duty cycle is present on the PCM data PID, the FPDM will double that (25% command from the PCM is 50% from the FPDM to the pump, 50% command from the PCM is 100% on command from the FPDM to the pump).
What you are seeing when you see the pressure raise and low, is the pump turning on and off... there is no pressure regulator. The PCM watches the fuel rail pressure sensor (FRP) and commands the pump on enough to achieve target pressure. Target is 40psi on cold fuel, and up to 55 psi on on hot fuel. pressure is NOT targeted on engine load like old returnless systems that used a vacuum operated pressure reg and return line.
Here is how it works. http://www.babcox.com/editorial/us/us120532.htm
Any codes? a dying fuel pump will set a P0191 (FRP performance) and often a P0148 (Fuel delivery error)... along with P0171 and P0174 (bank 1 and 2 lean)
If you don't have those... move on and leave the pump alone.