The common sprinkler valve always has a delay in closing the valve. The way it works is by relying on the water pressure build-up internally to stop the water flow. So even though the solenoid is released, it may take a few seconds for the water to stop flowing. Also, the lower water pressure you have the longer time it takes to fully stop the water flow. So if you are timing it according to when the water stops flowing, it will always be longer than the programmed time.