After I have fully charged my Sanyo NCR18650GA cells (4.2 volts) on my Nitecore D4 I take them off and instantly put them on again to double check their voltage and some of them have dropped fom their to 4.18 or 4.19 volts and some remain at their 4.20 volts.
Is this normal or something which may indicate a faulty battery or charger?
A minimal drop in cell voltage is fine, mine won’t stay at 4.20 either. 4.18-4.19 is fine.
However, I also have a cell that charges relatively quickly, charger stops at 4.20 and after removing the cell and using it in a light, it quickly goes to 3.6V. That cell is defective and has to be recycled.
Thankyou.
But why would a cell instantly loose as soon as you take them off the charger?
Some of us did some tests of various cells voltage drop over a couple of months you might find interesting.
T=2mn means minutes? 2 months?
I would assume t = months as he said they were tested over a couple of months not minutes
Yeah that’s months 
The voltage of a battery springs back a little when you stop charging it, like a literal spring.
This is why some types of batteries get trickle charged, however that’s not a good thing to do to lithium batteries.
A few tens of mV drop is normal.