Lead Acid batteries behave different to Lithium batteries during discharge and this difference on very significant. AGM 3911 tester use a load resistor to discharge the battery under test. As the battery voltage drops during the test from 13V ~ 9V it does not discharge at the constant rate. Instead for suggested 20A discharge test typically the discharge averages around 19A. LITHIUM M69C tester is constant current discharge i.e. electronic load, (not dependant on terminal voltage always load is maintained at 20A) thus give more accurate test result specially during final  stage of Li battery discharge. Also the software is different, it gives a capacity percentage instead of test run time.