It may have something to do with the current being put out by the device going to the reader. I have noticed this too. The Sony wall charger that came with my PRS950 has a output of 1500 mA @ 5 V. The wall chargers for my cel phone accessories are 500 mA, which is also the max by spec that can be drawn from a computer USB 2 port on a computer. It appears the readers are smart enough to recognize what they are plugged into and give different indicators to give a idea of how fast you are charging.
At least that's my guess.
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