It sounds like you're using a single gauge-type leak tester, like this
...and how to use here:
http://www.650motorcycles.com/LeakDownTester.html
A commercial tester typically uses two gauges; one to monitor the source air (100 psi for ease of calculating); the other to observe air pressure inside the cylinder.
In both cases, if you're starting out with 100 psi source air and the cylinder (@TDC end of compression) only holds 80 psi, the leakage is approx. 20%. That right?
It's possible for the oil rings and/or worn valve guide(s) to allow excess oil into the chambers yet compression still be good. Do you observe (hear) air leaking into the crankcase via the oil filler tube and other air escape paths?
Sherwood