I don't like articifial light, so my eyes are already used to half darkness. When I read in bed in the dark, where the PW is right in front of my face, I only have the light at around 4-5, otherwise it blinds me. I have, however, never seen any two PW to output the same brightness, so a combination of variation in device brightness and personal preference is probably at play here.
They must have used multicolor LED's and finally realized it. ~ S. attempting to explain the colorful blobs on Kindle Paperwhite screens.