A few years ago I carried out a compression test on an Lycoming 0-320 fitted to an R22 which had just flown in with no reported defects.
0/80, 0/80 0/80, 0/80
Obviously I was misusing the equipment so I tried again, more than once, same result.
So I got someone else to do the test, same result.
Then we tried another compression tester, same result.
We removed the exhaust and watched the valves moving through the ports.
The guides were so badly worn you could watch the valves jumping up onto their seats.
The only conclusion we could draw was that under power the inertia of the valve was keeping it where it should be, but on a static test the wear was obvious.
The best thing was the irate owner who could see no reason to work on the engine as it still had a 100 hours to go to overhaul!!!!!!!!!!!