Sometimes the dropping goes on so long(10-15 minutes) that the test is thrown out and the valve is opened and filled with water for a longer period of time. Pressure testers claim that it is because of air, but when I ask why does air in the valve cause a bad test, they can't answer me. Some technicians seem to think that it is because air is more compressible, but this doesn't make a lot of sense to me because some of our products are tested with nitrogen. I feel like it is possible that the system is not in equilibrium and air bubbles are sort of crawling around making their way to gather with a larger pocket of air in a stable pocket of the liquid.
I can think of two reasons why this might happen, though I don't know for sure.
First, air trapped in the test piece could be slowly dissolving into the water, which will change the overall density and hence the pressure. This doesn't happen when pure nitrogen is used because the nitrogen is not dissolving into anything.
Second, the mechanical parts of the valve could be getting adjusted to the pressure and might creep for a while until they balance the water pressure. Things like rubber valve seats and gaskets will do this.