If the tank has to be set at 2 psi below the pump. Why does the tank come pre-charged to 25 psi and the pump only produces 20 psi?

the pump comes ON at 20 and off at 40, you can ajust the pump pressure switch as needed.

This pump is factory preset to turn on at 20 PSI or less and turn off when it senses 40 PSI. When I installed mine, it performed like this based on the guage I installed in the line. The tank has 25 PSI behind the bladder which will not affect the working pressure in the water side of the pressure tank. As the pump forces water into the tank against the bladder and compresses the balloon the pressure will rise as it fills until it reaches 40 PSI then the pump will shut off. This will give you an operating supply of pressurized water depending on the volume of your tank it could be 5-10 gallons before the pump kicks back on.

most water well pressure tanks come with a factory pre-charge to accommodate the contingency of higher pressure switch settings, as it is much easier to expel the air than to add the air in the tank.