我想我大概自己找到答案了。也玩玩自问自答~
在看到界面上的亮度设定提示并查了一下KPW的背光技术之后，我觉得针对这个问题的结论是，背光的亮度范围并不是在工程和设计时他们要考虑的问题。具体说就是，在实现KPW背光的时候，顺序不是先在各种光线环境测试出适宜的背光范围，然后像编程那样设定对应的数字就好。工程实现时的首要考虑是如何让光线更自然更均匀。
KPW是在屏幕下方有四个LED，光线通过屏幕最外面的一层”nanoimprinted light guide”均匀地分布在屏幕上。四个LED应该是经过实验能够达到均匀效果的最小数量。四个灯加起来的最大亮度自然决定了背光的最高亮度。
It was actually very hard: We had to spend many hours in dark rooms to pick LEDs. We looked at samples and looked at for this batch here these are the best LEDS to match the characteristics of a white, very neutral color. We spent a lot of time looking at how many LEDs to put in, how hard to drive them, how bright we could get it without impacting the battery life. You don’t want someone to have to [consciously] manage their light, and say, I’d really like to have the light on but that’s gonna hurt my battery life.
至于在设计界面更是没有人考虑背光亮度的问题了。界面设计师甚至都没有意识到kindle e-ink屏幕的特殊性，心理模型仍是平时的LCD屏。见下图
参考资料：
Frontlighting, Examined
Kindle Paperwhite 2 Has Much Improved Frontlight, But Difference with Carta Screen is Marginal
HowStuffWorks “How the Kindle Paperwhite Works”
Kindle Paperwhite LED Lights Replacement
Problem solved! 开心^____^
— 完 —
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此问题还有 7 个回答，查看全部。
延伸阅读：
传统 LCD（CCFL 背光）与 LED 背光的 LCD 显示屏哪种更容易引起视觉疲劳？感觉 LED 背光屏在低亮度时的闪烁很严重。
为什么我们在强光下需要将手机亮度调高才能看清屏幕？
