Lithium ion/lithium polymer batteries use a Constant Current - Constant Votlage charge regime.
For the first part of the charge, the charger supplies a variable voltage to hold a constant charge current. The charge current is typically 1 x the capacity. So a 650 mAhr battery whould be charged at 650 mA for this phase.
This continues until the voltage reaches the maximum for the pack. For a lithium polymer this is 4.3 volts per cell. So phones and readers are typically since cell batteries, so at 4.2 volts, the charger holds that voltage, and the charge current reduces as the battery charges.
BUT, if the system is set up to use a lower maximum voltage, you get longer live (charge cycles) out of the battery. So many cell phones (and I assume readers) use a maximum votlage of 3.8 - 4.0 volts.
When the charge switches from Constant Current to Constant Voltage, the battery is over 90% charged. The remaining 10% can take as long or longer than it took to reach 90%.
So some devices stop the charge at 90% and call that 100%. Others will allow the charge to continue to a full 100%.