I think the battery gauge is separate from battery charger and somewhat unaware of eachother.

That said keeping a Li-Ion battery fully charged through trickle-charging is dangerous, so the charger stops once the battery is full. It tops up the charge periodically though. In real terms the charge level could vary by as much as 10 percent.

And on top of that the battery gauge making some informed guesses about battery's charge level.

BTW, if you only charge halfway, for example, you'll see the battery gauge adjusting its guess upwards for several minutes after you disconnected charger