I think what I am testing is knots, not rope. If I tie two knots in the same rope off the same reel bought at the hardware store, and load them in series with each other, then it should give me some idea of the relative jam resistance of the two knots. My test of hardware store rope may not apply to your EN564 cord purchased in Australia, but then your test of EN564 would not necessarily apply to my hardware store rope, which might be what I am really interested in.
Although all EN564 rope may meet certain specifications, that doesn’t mean all brands will be the same or even all samples from the same factory would be identical. Whereas, the hardware store rope should be pretty much the same a few feet away along its length.
Perhaps my assumption is wrong, and a certain knot could outperform another knot in one type of rope, but not in another type of rope. That would be interesting to know, but you’d have to test the knots in both types of rope to find out.