You can't go by the factory preset if your turn-on pressure of the control is set to 20 pounds. The tank needs to be set to 1-2 pounds LESS than the low turn-on pressure for your system or you will run out of water before the low point is reached and the pump turns back on. By setting the pressure in the tank high, you also decrease the amount of water the system can hold before it gets compressed to the upper set pont.
There is a one-way valve somewhere in the system that prevents water from being pushed/falling back into the pump when it shuts off (i.e., it should only allow water to come out of the well, not flow back down the pipe or at the minimum, keeping the water in the house!). If that leaks (sediment, seal, broken, etc.) the pump will come on until the upper pressure point is reached, turn off. While building up the pressure, it is compressing the bladder in the tank. If there are no leaks (either in the house or in the well system), then the pressure holds until you start to use some water. The bladder expands, the pressure drops, the control signals the pump to turn on when it gets to the low setpoint, and repeats.