I try not to assume anything anymore. I get phone calls from people who get upset that they leave their cylinder turned on and after a week or two they loose a couple hundred pounds. Many times that is from the valve itself, or overtime you do get some slight leakage. I get alarmed calls all the time that the gauge has lost some pressure after sitting overnight, turned off and are burning mad that their unit is wasting large amounts of gas. Some leakage is permissible. Leaving the cylinder turned off for an hour for lunch you can expect leakage when you come back. While in a perfect world you should expect the gauge to remain where it is, it just isn't so. There could be bits of rust from the tank lodging in the parts, or bits of sand, or corrosion that is blocking a full seal of the metal needle that seals in the solenoid, or the threads could not be sealing perfectly at the gauge or the unit or the valve on the cylinder is left in between closed and open and is leaking past the seal. Or it could just be worn. Many cyclinders are 50 or 60 years old or possibly more...so while valves are replaced, they get a good bit of abuse.