I would say that you have been very lucky Paul, there are 'after market nuts' and there are 'after market nuts'!
With the OEM wheel nuts you know that
a) they are made from the correct grade of steel every time and
b) almost certainly cold headed in the same factory as the ones originally fitted to the car.
With after market ones you do not have this . Ones bought one day might well be correctly specced both in material and manufacturing process (though unlikely if made to a price, in smaller quantities), while the next day, from a different supplier, they could well have been made from an inferior grade of steel and die threaded in some back street workshop in Pakistan. Less critical on a standard road car with full thread engagement, being pottered around by some old lady, but that is far from the case with your car 
Do not just assume that the after market nuts that you are fitting will hold up the same way as the ones previously on the car. The fact that they are plated already potentially modifies the steel structure, do research on 'hydrogen embrittlement and chrome plating'.
In the earlier days of Formula Ford racing there were some catastrophic accidents caused by failures of front wishbones that had been chrome plated to 'look pretty', where non plated ones had never failed before.........
