The battery meters (there are 3 in the N900, and you see the output of one of them) and the charger circuit don't talk to eachother.

Whenever the default meter that you see says "Full" when your device is on the charger, you know it's been full for a long time already.

As for the level of charge, there's a small variation in how full the battery is charged depending on the power consumption of the N900.

The biggest variation is in the inaccuracy of the default meter though, which isn't that good even with a fresh battery.