Having read this thread, I think there is an infinite number of variations of the displays within a certain range of tolerance in manufacturing.
Some are closer to the outer limits of that tolerance, borderlining on bad, and some bad apples (no pun intended) get through the QC, which is most surely done on the level of "batches of displays", not individual displays.
Is it a lottery? I'd say yes. It's the same with LCD televisions, in my experience: no two panels are the same.
At the end of the day, I think it is the tolerance of each individual customer that Apple has to deal with, case by case.