[Quoth]

You'll also find big variation if you have 20 of each.

Is this with front light on or off? They likely don't use the same LEDs and may not use the same lightpipe layer, though it's likely it is.

There is a lot of eink screen variation, unlike any other display technology ever. The lightpipe and how it's assembled etc make variation greater.

My Libra 2 and Libra H2O are identical under ambient light with LEDs off, but other people see significant differences.

The Kindle Colorsoft had a lightpipe issue too, with yellowness. Newer manufacture might not have it as much and Amazon claim to have reduced the issue in firmware.