Interesting part: Curiously, these drops in charge of battery don't come together with voltage drops, as you can see in the image. So, either the voltage sensor inside the N9 is not very precise or (most likely) the voltage is not the only indicator for the charge of battery...
The key to the bq27520-G2 ’ s high-accuracy gas gauging prediction is Texas Instrument ’ s proprietary Impedance Track ™ algorithm.This algorithm uses cell measurements, characteristics, and properties to create state-of-charge predictions that can achieve less than 1% error across a wide variety of operating conditions and over the lifetime of the battery.