I can't answer the question as to why the leads are failing. What I can say is that there are no settings for this test.
The test checks the Live and Neutral are the correct way round, and that there is no short circuit/open circuit. It is a straight Pass/Fail test, not a variable result with a cut off point.
Open circuit is quite often the fuse has blown - although it could also be a wire off.
There are only 4 possible results of this test. Good, Reversed L/N, Short and Open.
Have you checked the leads which fail to prove that they are not in fact failures?? The safest to start with would be one which is open. Does this power up an appliance??
Does a lead which fails for either fault always return the same fault - even if other leads are tested in-between??
If a large percentage pass with no problems, I would certainly start looking at the leads, not the tester.
Hope this helps.