I've seen this on some of my devices before, usually when the battery is very low. It's likely due to that fact that the % remaining is an estimate by the OS. As it nears closer to zero, it has a better idea how much is actually left and adjusts accordingly, especially if you've never software-calibrated the battery (charge full, discharge to zero, charge full).
It's also why a lot of times the last 5% of your battery seems to last as long as it took to drain the previous 10-15% - you actually had more battery than the OS estimated.
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