Fuel pressure anomaly?

When warm doesn't want to restart unless it sits for 30min approx. Fuel rail pressure reads 28psi key on and also when running. Spec for psi is about 55 so I normally suspect bad fuel pump. However, Leaving pressure test meter connected (reading that 28psi while running), I turn off engine. Over the course of about 5 minutes the static fuel pressure increases to about 55psi---again with the key off. Over the course of the next hour pressure will slowly drop to 10psi. Additionally, I managed to make a 2nd test meter connect at the fuel filter and got that same 28psi. The pressure regulator seems ok, no fuel leaking through the diaphragm. So, what makes the static fuel pressure rise from 28 to 55?