Where would you start to find the problem ?
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
[automerge]1589888196[/automerge]
So why do they make a rod that length if it's not good enough ?
They make them because there is a demand for the product and its easier to transport and use an earth rod in 4' sections than to try and install an 8' rod or longer in one hit.
Also earth rods aren't only manufactured for permanent install as part of an electrical installation, they have other applications in temporary and permanent installations.
The really thin non-extendable rods are good for electric fence installs, temporary generator supplies and many other applications but they are completely useless for permanent installations.
Manufacturers aren't required to produce products for your specific application, they aren't concerned with BS7671 or best practice, they just make whatever product they can sell.
It is up to you to ensure that you select the correct materials for a job and install them in accordance with best practice.