The battery never reports how much is left. The phone only monitors the battery voltage and takes a guess as to how much capacity is left based on battery profiles.
Not directed at you, but there is a very common misconception that batteries tell devices how much juice is left. It's actually just a tuned guess as to how much capacity is remaining. There's lots of factors that I won't go into here, but it gets very complex.
The drop in reporting may very well be the reboot, where the device has to run the screen, processor and memory at full capacity during the boot sequence. This drains the battery, or at least pulls the voltage down enough so the built in voltage curves guess there was a drop.