Weird fuel pressure readings

[OneLethal87GN]

About a week and a half or so ago I noticed that when I had previously shut the car off the fuel pressure was @ 25 psi. A few days later I checked it and it was @ 15 or so. The next day it was reading slightly above 20 psi. Can temperature changes cause this? The gauge is a K-B hood-mounted gauge. By the way, the car wasn't cranked at all between the different readings. It didn't even have a battery in the car at the time. I removed it after I first shut it down. What could cause up and down fluctuations in fuel pressure on a car that doesn't even have a battery in it?

 

[OneLethal87GN]

Is this just a very rare problem? I'm sure of the readings. I told my g/f how weird it was to go down one day, then up the next without even having a battery in the car. I could see if I had ran the car or something but this is too weird.