your cable has many parts. the long cable itself. the connector to the USB port. the connector to the micro USB port

all these build up the entire cable "system". as shown in the above water example, as long as 1 of the part is not in optimum condition, it will drag the entire system down

as to your experiment, yes you try to use the same setup as close as possible but once you unplug and plug in again, you already change the settings a bit, so can affect the final current

just one thing to note

even those ori cable that can achieve 1800 constantly will also have this problem (unplug and after some time plug again will have different readings) but in my opinion, i think the ori cable has a higher current carrying capacity than 1800, so you can't see the difference.

but those not so optimum cables because it can't reach the max of 1800, any slight change, you see it

Example (assuming charger can reach 3A max current)

If ori cable max current carrying capacity = 2300ma, when you plug and unplug it will varies between 2000ma to 2300ma. However because Note2 allows only max 1800ma, you can't see the change

If non optimum cable max current carrying capacity = 1500ma, when you plug and unplug it will varies between 1200ma to 1500ma. But now you can see the change because it is below the max Note2 allowed current

Hope it make sense