Unless, um, something's wonky with the battery or with the bits that read its levels, which is exactly what all of this discussion is centered upon. Thanks for the cynical skepticism though (?), it's helpful.
Also, I just watched 45minutes of Netflix with the brightness all the way up. Brought it down 20%. Another 5% after sitting about 10-15 minutes. It does seem to be the full brightness that brings it down. I'm just going to let it die off and then plug it in for the night.
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Mine's
doing a little worse than that.