I would start with logic, if the continuity reading is good but the Zs is not then logically it is going to be something that is unique to the Zs test that's causing the problem.
So that rules out the circuit itself, so that leaves the connections between the CU earth bar and the means of earthing, the connections of the live meter tail, the tester settings, the tester connections.
Start ruling these things out in a logical order, check all of your connections, retake the measurement after checking all test lead connections and the settings of the tester.
Check that you aren't using an 'anti trip' or low current test unnecessarily. If you do need to use an 'anti trip' test then this may account for the discrepancy, test a couple of circuits using the alternative test method and a high current test to see if this clears it up