[Brightcolours]

I disagree.
This is why for certain lenses, you will see people raving about it while some people would say they suck.
It's happened and it will keep happening unless QC is very tight.
Of course, it depends on the lens model.
Lensrentals.com have shown this: a good sample of a an inferior lens could give better overall MTFs than a bad sample of a better lens.

Lemsrentals has never said that the differences are not due to centering issues. 
So what other "QC issues" do you want to put forward?