This is what I noticed to be common amongst most units.
The edge where the smart connector resides is slightly darker in uniformity than the other side. On some units, the backlighting is also uneven with spots and patches. The easiest way to find out is to read from your iPad in normal portrait view, then flip it upside down where the home button is at the top, then you may see what I'm talking about.
dL
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Some units are top left. Some units are mid left. Very few units are bottom left. Bottom line is the left side is slightly darker on all units, but only a trained eye can see. This is definitely not as bad as the book spine from the Air series.
dL