Never compare numbers from one compression gauge to another. There are three good gauges in my shop, all of which read differently. There are a couple of things people often miss on a compression test. The engine should be warm. Loosen the spark plugs a turn. Put the wires back on and start the engine. Zap it a couple of times, then shut it off and pull the plugs. This is to blow out the little bits of carbon that break off when the plugs are removed. This is the number one cause of inaccurate readings, as those chunks of carbon land under a valve and hold it open. Now remove the plugs, screw the tester in, floor the throttle (this isn't real important on a Volvo, but a car with Webers or something with little air in the manifold it can make a huge difference.) and crank the engine through 4 "puffs" only. If you watch the gauge, it should jump to within probably 80% of its final reading on the first two puffs. Slow rising means poor compression rings. A low reading that jumps up to its max fast is a leaking valve. The wet test will confirm rings, but even a healthy engine will rise some on a wet test. Compare cylinder to cylinder. A 15 psi difference is really not significant, but much more and you are looking at trouble. Look at the whole picture, too. Are the plugs carboned up? Lots of blowby thru the breather? Rougher idle than you expect out of a volvo? All of these are signs.