Saab 9-3 low cylinder compression, in 1 cyl, any advice?

1999 Saab 9-3 Turbo, 200K miles. Car starts, runs and drives fine. There is a very minimal "miss" when the engine is running, and a slight lack in acceleration, not enough to be noticable unless you had driven the car when it was new like I had, it just used to be more peppy, I thought it was due to age and mileage. As it was in the mechanics for a fuel pump, they found that 3 cylinders were at 120psi, and 1 was at 80psi. It doesnt burn oil, doesnt lose coolant. and does not overheat, so Im wondering what could cause this? Mechanic says I need to address this ASAP, Does anyone have any suggestions as to what this could be? I assume its not a head gasket because Im not losing coolant and its dont overheat, even with the a/c on, and it doest smoke or use excess oil. Ive been told so many different things and Im not mechanically inclined, went to get a 2nd compression check, and it is indeed 80 psi while the others are 120, so the mechanic isnt lying!