I`d say mostly because of this (posted on Amazon`s site even before PW was available to buy):
Quote:
Perfectly Balanced Whiteness
We worked on Kindle Paperwhite for over two years to perfect the uniformity of the built-in light, flattening out a fiber optic cable into a sheet, and nanoimprinting to ensure perfectly even distribution of light. Our design uses nanoscale optical diffractive patterns to enable tight control over the direction of the light. Enjoy reading with exceptional lighting uniformity and evenly balanced whiteness across the entire display.
http://g-ec2.images-amazon.com/image...paperwhite.jpg
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That`s what people were pre-ordering, but that wasn`t what they were getting in the end...