Why does an expansion tank need to be pressurized with the same pressure as the water supply?
Normally a pressure tank is charged to a few psi less than the cut in pressure of the pump. The reason is that this allows the water to be pushed out of the tank with out having to start the pump as often. If you are on city water and you have an expansion tank 3-5 psi less than the normal water pressure allows back pressure from the water heater to be absorbed. I have seen that water heater cycling actually can cause an increase in water usage because the water meters don't deduct if there is a reverse flow. Having the tank at a lower pressure allows for the maximum of space in the tank.